This question tests your knowledge of what an unsaturated lipid is, as well as your ability to obtain structural information from IUPAC names.

Firstly, a polyunsaturated lipid is a long carboxylic acid hydrocarbon chain that features multiple unsaturations, or double bonds. Remember from nomenclature that the stem "-en" indicates double bonds, or alkenes, while "-an" indicates alkanes, or fully saturated hydrocarbons. The relevant stems in the answer choices are italicized below:

I. (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid

II. Octadecanoic acid

III. (9Z,12Z)-octadeca-9,12-dienoic acid

IV. (9Z)-9-Octadecenoic acid

As you can see, "trien" and "dien" in I and III indicate these lipids are polyunsaturated, while "an" in II indicates a fully saturated lipid and "en" in IV indicates a monounsaturation. The lipids are drawn below as well, with the alkenes in polyunsaturated lipids circled in red, and the alkene in the monounsaturated lipid circled in green.

The pictured structure represents arachidonic acid due to the 20-carbon carboxylic acid chain with characteristic unsaturated (double) bonds after carbons 5, 8, 11, and 14.

Fatty acids can be labeled numerically from 1-X, or they can be labeled in reverse starting with omega-1 as the terminal carbon.

Oleic acid is a fatty acid consisting of 18 carbon molecules and a single unsaturated (double) bond after carbon 9, as pictured.

A triglyceride consists of three fatty acid chains bound to a glycerol backbone via ester bonds, as shown by the pictured structure.

The reaction described is an esterification in which water is a product—as is characteristic of a condensation reaction—and in which an ester bond is formed to connect the fatty acid chain and the glycerol molecule.

This question is a little bit tricky. At first glance, we would jump to the conclusion that this molecule is a long hydrocarbon chain attached to a carboxylic acid and, therefore, a fatty acid. The premise is correct. However, our conclusion is false because, by convention, a fatty acid must contain a carbon chain of at least 12 carbons. The given formula does not match the description for a steroid or triacylglycerol. The correct answer is none of these.

The primary biological function of a fatty acid is to serve as a medium for the derivation of other important lipids such as waxes, triacylglycerols, phospholipids, and other necessary lipids.

A triacylglycerol is derived from the hydrolysis of glycerols and fatty acids. Triacylglycerols have hydrophobic hydrocarbon tails and hydrophilic heads (amphipathic). The hydrophobic tails dissolve into dirt/grease on the skin while the hydrophilic tails work with running water to suspend and remove the dissolved impurities.

The correct answer is the lipid, wax. Lipids can often be identified based on their structure alone. The general structure of wax is an ester with two long carbon chains on each end of the ester, as seen in this problem. Additionally, fatty acids can be identified simply as long carbon chains with a carboxylic acid on one end, while the general structure of steroids can be seen as four connected hydrocarbon rings.