Which of the following molecules is most likely to serve as a building block for proteins?

Amino acids are the molecules that serve as the fundamental building blocks for proteins. Each protein is made up of one or more chains of amino acids, and the specific sequence of these amino acids determines the protein's unique structure and function. There are 20 different amino acids, each with distinct properties, and they link together through peptide bonds during a process called translation in protein synthesis.

In contrast, sugars are primarily involved in energy storage and supply in the form of carbohydrates, nucleotides are the building blocks of nucleic acids like DNA and RNA, and fatty acids are components of lipids, which serve primarily for energy storage as well and structural roles in cell membranes. Therefore, while all these molecules are important for various biological functions, amino acids are specifically critical for the formation of proteins.