The part of the alimentary canal where protein digestion begins is the
a. mouth.
b. esophagus.
c. small intestine.
d. stomach.

d. stomach.

Explanation:

In humans, proteins start to be digested in the stomach, an organ that can expand when full of food and fold up when empty.  Proteins are denatured as a consequence of the interaction with gastric secretions, specially pepsin that can only activate in acid environments. The next step will be the small intestine when those proteins, which converted to chains of polypeptides can be fully digested.