Amain job of hcl is to begin digestion. carbohydrate fat protein

The answer to this is protein.

Hydrochloric acid in digestive system is found in stomach, which is the only main digestive organ with an acidic pH 2.

In the stomach, only one kind of the primary food substance is chemically digested, which is protein. It is digested with the help of enzyme Pepsin, which breaks protein chains into polypeptides.

After that, the other organs of the digestive system has more of a neutral to slightly alkali condition, which is not related to hcl. Therefore, the answer is protein.

D. Adenine, Ribose and three phosphate groups.