Explanation:
The correct answer is D: Proteins.

Proteins are primarily digested in the stomach. The stomach secretes gastric juice containing hydrochloric acid (HCl) and the enzyme pepsin, which begins the breakdown of proteins into smaller peptides.

Here’s why the other options are incorrect:
- A: Carbohydrate: Carbohydrates are mainly digested in the mouth by the enzyme amylase and in the small intestine by other enzymes such as pancreatic amylase.
- B: Fats and oil: Fats are mainly digested in the small intestine, where bile emulsifies the fat, and pancreatic lipase breaks it down.
- C: Fats and proteins: While proteins are digested in the stomach, fats are primarily digested in the small intestine, not the stomach.
- E: Carbohydrates and fats: Carbohydrates and fats are not digested in the stomach; they are digested in the mouth (carbohydrates) and the small intestine (fats).

Therefore, the correct answer is D: Proteins.