Answer:
A. a protein
Explanation:
Proteins function as enzymes in most of the catalyzing reactions, that increases the rate of reaction within cell.
Proteins are consist of multiple numbers of amino acids and the twisting and folding amino acids provide a unique identity to enzymes. Not all the proteins are enzymes but all the enzymes are proteins.
For example: Trypsin, Pepsin, and Lipases.
Hence, the correct option is "A. a protein".
