Each polypeptide is a protein that has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be:

The primary structure of a protein refers to the linear sequence of amino acids in the polypeptide chain. The primary structure is held together by covalent bonds such as peptide bonds, which are made during the process of protein biosynthesis or translation. The primary structure of proteins tells us about the sequence of amino acids in which they are linked. Option A is correct.