Amino acids are best described as:

Amino acids are best described as the building blocks of proteins because they are the fundamental units that combine to form proteins in the body. Each protein is made up of one or more chains of amino acids, linked together by peptide bonds, and the specific sequence of these amino acids determines the structure and function of the protein. Proteins play critical roles in the body, including serving as enzymes, structural components, and regulators of biological processes.

In contrast to the correct answer, complex carbohydrates refer to larger molecules composed of sugar units and are not directly related to amino acids. The basic units of nucleic acids are nucleotides, not amino acids. Nucleotides are the building blocks of DNA and RNA, crucial for genetic information. Essential vitamins for metabolism are organic compounds that the body needs in small amounts to function properly, but they are not connected to amino acids or protein synthesis. Thus, the characterization of amino acids as the building blocks of proteins uniquely defines their primary biological role.