Final answer:
The purpose of a functional group is to assist molecules in chemical reactions, influencing their properties and interactions with other chemicals, thus playing a crucial role in the formation and function of biological molecules.
Step-by-step explanation:
The purpose of a functional group is C. to assist molecules in chemical reactions. Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They influence the properties of molecules and how they interact with other chemicals. For example, they can make a molecule hydrophilic (water-attracting) or hydrophobic (water-repelling) depending on their polarity. Functional groups such as hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl are crucial in the formation and function of biological molecules like DNA, proteins, carbohydrates, and lipids. By affecting charge, polarity, or the ability to form hydrogen bonds, functional groups can significantly alter the behavior of the molecule they are attached to.