Le Chatelier's principle states that a system will shift in a particular direction in order to reduce introduced stress to the system. In other words, if something is added to one side of the equation, the other side will consequently increase in order to restore equilibrium.

The above reaction is exothermic, meaning that heat is released from the system. As a result, heat is considered a product.

By removing nitrogen or hydrogen gas, the system would shift to the left in order to create more reactants. If temperature is decreased, the system will create more heat by shifting to the right. Not only will this create more heat, but it will also create more ammonia, as they are both products of the reaction.

We can set up the equilibrium expression by placing the products, raised to the power of their coefficients, in the numerator and reactants, raised to the power of their coefficients, in the denominator:

We are given the value of the equilibrium constant and the equilibrium concentration of . Using stoichiometric coefficients, we can determine that a variable concentration, , of  will be present at equilibrium, and  of  will be present at equilibrium. Plugging in:

Simplify:

Now we can solve for  (which is the equilibrium concentration of , as we assigned) by dividing both sides by 4 and then taking the cube root.

Rounding to 2 sig figs (to match the two numbers we were given by the problem), we get:

Addition reactions follow the general pattern:

When magnesium combines with oxygen gas, it results in a compound with both of them in it. This makes it a simple addition reaction. In general, addition reactions have a greater number of reactant compounds than product compounds.

The other answer choices correspond to the reactions below:

Decomposition reaction:

Single-replacement reaction:

Double-replacement reaction:

Addition reactions occur when two molecules combine to form a larger one, leading to a release of energy. Addition reactions result in the synthesis of a new molecule, giving them their second name: synthesis reactions.

Many synthesis reactions can also be classified as dehydration reactions, if water is formed as a byproduct. One example of this is the addition of amino acids to form protein chains. Hydrolysis reactions use water as a reactant and are used to split molecules apart (decomposition).

Dissociation reactions occur when one molecule is divided to form two smaller ones, leading to a decrease in energy. Dissociation reactions result in the break down of a large molecule to form smaller products, giving them their second name: decomposition reactions.

Many decomposition reactions can also be classified as hydrolysis reactions, if water is used as a reactant. One example of this is the breakdown of glycogen to form glucose monomers. Dehydration reactions release water in order to form a new bond, building a larger molecule from smaller reactants (synthesis).

When one molecule is converted into two or more smaller molecules, the reaction is considered a decomposition reaction. In this example, none of the elements under go a change in oxidation number, so this is not an oxidation-reduction reaction. (Calcium is always , oxygen is always , and carbon remains  throughout the reaction.)  

Dissociation reactions usually entail a salt or ionic compounds dissociating or splitting into its components. In this case,  where AB is made up of the components A and B. This reaction is ideally reversible even though the question does not provide the reversible arrows. 

The options that include a third species, would be incorrect, as there are only two species in the original compound that is dissociating. 

A reaction that has both reduction and oxidation half reaction is called a redox reaction. It involves one or more atoms gaining electrons (reduction) and one or more atoms losing electrons (oxidation). Recall that single displacement reactions involve the replacement of an element in a compound with another element. An example of single replacement reaction is shown below:

In this reaction, a sodium atom replaces a calcium atom in calcium sulfate. If we calculate the oxidation numbers for sodium and calcium, we can see that sodium loses an electron (the oxidation state goes from  to ) whereas the calcium ion gains two electrons (goes from  to ); therefore, this is a redox reaction. All single displacement reactions follow this general trend and are characterized as redox reactions.

Reaction of sodium chloride and calcium sulfate is as follows:

If we calculate the oxidation state of each atom we will notice that oxidation number doesn’t change; therefore, this isn’t a redox reaction.

By looking at the reaction, we see that the cations for the reactants have their anions switched in the products. This means that the reaction follows the general outline:

This is an example of a double-replacement reaction.

Addition reactions convert two reactants to a single product, while decomposition reactions convert a single reactant to multiple products. Single-replacement reactions only switch one cation-anion pair.

In a double displacement reaction, or double replacement reaction, opposite ions combine. What this means is that the cation of one compound will recombine and bond to the anion of the other compound and vice versa. 

In this example, we have to remember the ideal convention of cations being written before the anion in the compound. So in AB, we can presume A being the cation and B being the anion. The same goes for CD - C is the cation and D is the anion.

With this in mind, we can now easily see how the replacement would be possible. 

If A is a cation, and was originally bound to B (anion), the only other anion it is left with to bind is D. If C is a cation, and was originally bound to D (anion), it is only left with the option of binding to the now-free B anion. 

That's why it is: