The correct answer is: A. Mouth
Digestion of protein begins in the mouth with the mechanical breakdown of food by chewing. Saliva contains an enzyme called amylase, which begins to break down starch into smaller molecules.
The pharynx is a muscular tube that connects the mouth to the esophagus. It does not play a role in digestion.
The stomach is a muscular sac that stores food and begins the chemical breakdown of proteins. The stomach produces hydrochloric acid and enzymes that break down proteins into smaller molecules.
The liver is a large organ that performs many important functions in the body, including the metabolism of proteins. However, digestion of protein does not begin in the liver.