Which of the following best defines the structure of nucleotides?

The structure of nucleotides is indeed best defined by a combination of a sugar, a phosphate group, and a nitrogenous base. Nucleotides are the building blocks of nucleic acids, such as DNA and RNA. Each nucleotide consists of three components:

1. A five-carbon sugar (ribose in RNA and deoxyribose in DNA).
2. A phosphate group, which contributes to the acidic nature of nucleic acids and creates the sugar-phosphate backbone.
3. A nitrogenous base (adenine, thymine, cytosine, guanine in DNA; adenine, uracil, cytosine, and guanine in RNA), which encodes genetic information.

This specific combination allows nucleotides to link together to form long chains (polymers), creating the structure of nucleic acids that store and transmit genetic information in living organisms.

In contrast, the other choices pertain to different biological macromolecules: fatty acids and glycerol are components of lipids; amino acids are the building blocks of proteins; and monosaccharides, disaccharides, and polysaccharides relate to carbohydrates. Each of these groups has distinct structures and functions that do not describe the nucleotide structure accurately.