Function of HCL in stomach is to
A. Kill microorganisms of food
B. Facilitate absorption of food
C. Activate pepsinogen to pepsin
D. Both A and C

Hint: Digestion of food starts in the oral cavity in humans, continues in the stomach, and is completed in the small intestine. The food undergoes no change in the pharynx and the oesophagus. These regions only convey food from the oral cavity into the stomach by swallowing.

Complete step by step answer: The presence of food in the stomach stimulates the gastric mucosa to produce gastrin hormone. The latter is absorbed in the bloodstream and stimulates gastric glands to secrete gastric juice.
-The goblet cells present in the lining of the stomach secretes mucus. Also, the pyloric tubular glands secrete mucin which lubricates and protects the epithelial surface from excoriation by HCl and digestion of enzymes. The chief cells, also called peptic cells and zymogen cells, secrete pepsinogen (proenzyme) which is an inactive precursor of the pepsin enzyme.
-In the presence of HCl, pepsinogen gets converted to an active form, i.e., pepsin. The activated pepsin by autocatalysts activates more pepsinogen to pepsin. The pepsin enzyme is more pepsinogen to pepsin. -This pepsin enzyme is the principal protease of the stomach which hydrolyses proteins into peptides.
Probably, chief cells also secrete another enzyme in the infants in an inactive form, i.e. prorenin.
-The latter gets activated to rennin in the presence of HCl and hydrolysis the soluble milk protein casein to paracasein and whey protein. HCl acid also kills the microorganism present in the stomach.
So, the correct answer is option D, i.e., Both A and C

Note: Gastric juice also secretes gastric lipase enzyme. It converts a negligible amount of fats into monoglycerides and fatty acids because the enzyme is sensitive to free acid and is destroyed by HCl. Also, the stomach lacks a fat emulsifying agent.