This question tests ones knowledge of carboxylic acid derivatives and how different groups effect reactivity. The order is 3, 1, 4, 2 and this is because of the electronic effect. Acid chloride (3) is first because of its electron withdrawing group making it most reactive. (1) is a weakly donating group and (4) is a normal strength donating group and (2) is a good electron donating group. 

The correct answer is D.

The molecule pictured above is an aldehyde because it contains an oxygen double bonded to the last carbon on the carbon chain. A contains a amide group, B contains a ketone, and C contains a carboxylic acid group.

A carbonyl functional group takes the form of . This leaves the carbonyl carbon to form two remaining bonds with various substituents. If one of these substituents is a hydrogen, the molecule is an aldehyde (the carbonyl group is at the end of the carbon chain). If neither substituent is a hydrogen, the molecule is a ketone (the carbonyl group is not at the end of the carbon chain).

An acid contains the carboxylic acid functional group, which takes the form of . Carbonyl groups do not contain the .

Amino acids contain the carboxylic acid functional group () and an amine functional group (). The description in the question does not fit this molecule.

The prefix "form-" indicates a  group. This group by itself is known as the "formate" anion, and its protonated acidic form is known as "formic acid." This is a very common organic acid and it is important to know the name of this acid.

The molecule's longest carbon chain has 4 carbons (thus, "but-"), and the lack of carbon-carbon double bonds makes it an alkANE (thus "butan-"). The presence of a carbonyl () group that is not at the end of the chain means that the molecule is a ketone (thus "butanone"). There is no other way to make a ketone on a carbon chain consisting of 4 carbons, and therefore a locant is not needed when naming this compound.

The molecule's longest carbon chain has 5 carbons (thus, "pent-"), and the lack of carbon-carbon double bonds makes it an alkANE (thus "pentan-"). The presence of a carbonyl () group that is not at the end of the chain means that the molecule is a ketone (thus "pentanone"). Numbering the carbon chain so that the carbonyl is on the lowest possible carbon will put the carbonyl on carbon 2 (Thus "2-pentanone").

The prefix "acet-" or "aceto-" indicates a  group. This group by itself is known as the "acetate" anion, and its protonated acidic form is known as "acetic acid." This is a very common organic acid and it is important to know the name of this acid.

The longest carbon chain is  carbons long (thus ""), and the lack of double bonds makes it an alkane (thus ""). The highest priority functional group is the carboxylic acid group, and the suffix of the IUPAC name should reflect the highest priority functional group (thus ""). Because the carboxylic acid group is assumed to lie on carbon number  there is no need to designate a locand for this functional group. The other substituents of this molecule are the methyl groups on carbons  and  when numbering the chain from left to right. Thus, the name is .

The molecule is an amide (a molecule of the form ). The parent chain is the unbroken chain containing the carbonyl () group, including the carbonyl carbon. Thus, in the molecule shown, the parent chain consists of four carbons with no double/triple bonds (giving the root ""). The most important functional group is the amide group, so the suffix to the molecule's name is  (thus ""). Furthermore, the carbon chain automatically labels the carbonyl carbon as carbon number  so there is no need to label the amide functional group with a locand (it's redundant information). Alkyl groups attached to the amide nitrogen are given the locand "" to show that they're on the nitrogen. Thus, the molecule is called .

When naming an organic compound by the IUPAC, it's best to first start by identifying the functional groups present.

In this particular case we have:

A ketone here:

and an ethyl group in the middle:

Next, we should identify what functional group has the highest priority, as that will form the base name of the compound:

According to IUPAC convention, Carboxylic Acids and their derivatives have the highest priority then carbonyls (ketones in this case) then alcohols, amines, alkenes, alkynes, and alkanes, so in this case the ketone group has the highest priority and therefore makes up the name of the base compound.

Next, we want to number the longest carbon chain with the highest priority functional group with the lowest number. In this case this means we want the ketone to be carbon number #2, which is the lowest number it can possibly be so let's start there and number the carbon chain.

You should get something like this:

Now that we have numbered the carbon chain we can begin our naming.

Let's start with the base name:

According to IUPAC convention the base name for a ketone compound is -one, so in this case we have a octan-2-one.

We also have an ethyl group at carbon five. This gives us 5-ethyloctan-2-one, which is our final answer.

Now let's look at the wrong answer choices:

1) 5-propylheptan-2-one is wrong because the longest carbon chain has 8 carbons and following the path of the ethyl group as part of the main chain prevents naming being done for the longest possible carbon chain.

2) 5-ethyloctanal is incorrect because it mistakes the ketone for an aldehyde as the suffix -al refers to aldehydes which are RCHOs that can only be carbon 1.

3) 5-propylheptanal is wrong because the longest carbon chain is 8 carbons long, not 7, and this compound isn't an aldehyde.

4) 4-ethyl-octan-2-one is wrong because the ethyl is at carbon 5 and not carbon 4.