For this question, we're presented with the molecular structure of some unknown compound, and we're asked to identify which answer choice offers the most likely identity for this compound.

Since there's no way for us to know exactly what this molecule is (unless you have it memorized!), we'll need to look at each answer choice.

Insulin is a peptide hormone whose main function is to allow cells in the body to take up glucose from the blood.

Epinephrine is also a hormone, but in addition it can act as a neurotransmitter. One of epinephrine's main roles is in the fight-or-flight response, where it acts to mobilize the body's energy reserves in preparation for a demanding situation.

NADH is a cofactor involved in a variety of catabolic pathways, such as glycolysis. Its main role is to act as a carrier of high-energy electrons, which it can donate into the electron transport chain to produce cellular energy in the form of ATP.

Methionine is one of the twenty amino acids found in organisms. It thus acts as a key constituent in many proteins.

Cortisol is a steroid hormone, often called the "stress hormone." It plays a variety of roles in the body, some of which influence the body's metabolism of glucose (cortisol is a type of glucocorticoid hormone).

When looking at the structure given in the question stem, we can see that this is a pretty big molecule! While we don't need to have the exact structure of cholesterol memorized, we can recall that cholesterol is a big molecule that has multiple rings in its structure, just as this compound does. Thus, we can be reasonably sure that this compound is most likely a derivative of cholesterol. Many derivatives of cholesterol function as steroid hormones, which are largely hydrophobic molecules that are able to transverse cellular membranes. Since we know that cortisol is a steroid hormone, we can assume that this structure most likely represents cortisol.

This question is presenting is with the molecular structure of some unknown compound, and is asking us to identify the most appropriate classification for it.

From the picture, we can see that this structure has a carbon backbone. On the carbon backbone, we have three long hydrocarbon chains attached to the backbone via an ester linkage. Thus, this compound is a triglyceride (also known as a triacylglycerol).

From our understanding of biochemistry, we know that triglycerides are a type of lipid, thus making this the correct answer.

The structure shown is not a protein or a nucleic acid, as both of these compounds contain nitrogen or phosphate as part of their molecular structure, respectively. Proteins are linked by a characteristic series of amino acids, each of which has a unique side chain group. Nucleic acids, on the other hand, contain a sugar-phosphate backbone connected by phosphodiester linkages, along with nitrogenous bases.

Furthermore, this structure is not a carbohydrate, as these structures generally have a carbon:hydrogen:oxygen ratio of .

An isoprene unit  repeats many times to form molecules called terpenes. Squalene is a triterpine - made from six isoprene units. Vitamin A is known as a terpinoid because it is derived from squalene.

Arachidonate is a precursor for many biologically important molecules. If acted on by lipoxygenases it can be converted to leukotrienes. If acted on by prostaglandin synthase and then subsequently by another (or several other) enzymes it can be converted to prostacyclin, prostaglandins, or thromboxanes. However, it is not a precursor for cholesterol.

Essential fatty acids cannot be produced by the body and need to be obtained thru diet. Linoleic acid is unsaturated, has 2 double bonds while alpha-linolenic acid has 3 double bonds. Both have a backbone structure of 18 carbons.

Phospholipids have two fatty acid chains, with a length of 14-20 carbon atoms each, that attach to the carbons of the glycerol molecule. Examples of glycerophospholipids are phosphatidyl glycerol (PG). Examples of sphingolipids, like sphingomyelin, have choline, phosphate, sphingosine groups and only one fatty acid.

Glucose and fructose have the same constituent atoms, so they have the same molecular mass (about ). Glucose exists in aqueous solution in an equilibrium of open-chain and several cyclic isomer forms, the most common of which is pyranose, but also one of which is furanose. Upon forming a ring structure, glucose may take one of two anomers, alpha or beta. The relative proportion of the mutarotated beta form is increased versus the alpha form because the beta anomer is such that all non hydrogen substituents are in the equitorial position. Glucose can indeed be isomerized into fructose; this is part of glycolysis.

Sucrose is the molecule shown below(in Haworth projection). 

We recognize that it's a disaccharide, and that the first molecule is -D-glucopyranose. The bond to the second monosaccharide (fructose) is tricky because the glycosidic bond is going to the anomeric carbon of fructose. Therefore, this is a 1-2 linkage. Finally, since the anomeric carbon of fructose is pointing in the same direction as the 6th carbon, this is a -D-fructopyranose.

Lactose is made by joining a  galactose to a glucose via a  carbon pathway. Both sugars are D-sugars. 

A set of anomers will differ in the configurations of their carbonyl carbons only while a set of epimers will differ in configuration in only one carbon that is not the carbonyl carbon.