Which of the following describes a protein?

The correct choice describes a protein as an organic compound composed of polypeptides formed from amino acids. Proteins are fundamental macromolecules in all biological systems and play crucial roles in a variety of cellular functions. The building blocks of proteins are amino acids, which link together in specific sequences to form polypeptides. These polypeptides fold into unique three-dimensional structures, which determine their specific functions, such as catalyzing biochemical reactions (enzymes), providing structural support, and facilitating communication within and between cells.

By contrast, the other options do not accurately characterize proteins. Nucleic acids, such as DNA and RNA, are composed of nucleotides and are involved in the storage and transmission of genetic information. Simple sugar molecules, known as carbohydrates, serve primarily as energy sources or structural components but have no relation to protein structure. Lastly, nucleotide polymers consist of nucleotides linked together and form nucleic acids, which is distinct from the amino acid composition of proteins. Thus, the accurate depiction of proteins lies in their polypeptide makeup and the role of amino acids in forming them.