Proteins are large, complex macromolecules (polymers) made up of long chains of smaller repeating units (monomers) called amino acids. These amino acids are linked together by peptide bonds to form a polypeptide chain, which then folds into a specific 3D shape essential for its function.

A. glycerol and C. fatty acid are the building blocks that combine to form lipids (fats and triglycerides), not proteins.

B. carboxylic acid is a functional group present in all amino acids, along with an amino group. However, the entire amino acid molecule, not just the carboxylic acid part, is the fundamental unit of a protein.