Correct Answer: D

Rationale: The correct answer is D: Hydrochloric acid. Hydrochloric acid in the stomach plays a crucial role in converting pepsinogen into its active form, pepsin, which is necessary for digesting proteins. Bile (Choice A) is involved in the emulsification of fats, not in the activation of pepsinogen. Gastrin (Choice B) is a hormone that stimulates the secretion of gastric acid but does not directly activate pepsinogen. Secretin (Choice C) is a hormone that regulates the pH of the duodenum but is not responsible for the activation of pepsinogen.

Correct Answer: D

Rationale: The correct answer is the amino group. Amino acids consist of an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable side chain attached to a central carbon atom. The amino group is essential for the formation of proteins. The carboxyl group is not the base component but another important functional group in amino acids. Carbon chain and radicals are not specific chemical structures that form the base component of an amino acid.

Correct Answer: D

Rationale: The correct answer is D: Hydrochloric acid. Hydrochloric acid in the stomach plays a crucial role in converting pepsinogen into its active form, pepsin, which is necessary for digesting proteins. Bile (Choice A) is involved in the emulsification of fats, not in the activation of pepsinogen. Gastrin (Choice B) is a hormone that stimulates the secretion of gastric acid but does not directly activate pepsinogen. Secretin (Choice C) is a hormone that regulates the pH of the duodenum but is not responsible for the activation of pepsinogen.

Correct Answer: C

Rationale: Probiotics are live bacteria that are beneficial for gut health and are often taken as supplements. They are not indigestible carbohydrates (Choice A), antibiotics (Choice B), or commercial fiber supplements (Choice D). Probiotics contain live strains of good bacteria that help maintain a healthy balance in the gut microbiome.

Correct Answer: C

Rationale: The correct answer is C: mucus. Mucus forms a protective barrier that prevents the stomach and intestine from digesting themselves. Pepsinogen (choice A) is an inactive form of pepsin, a digestive enzyme, and does not protect the stomach lining. Bile (choice B) aids in the digestion and absorption of fats but does not protect the stomach lining. Fat (choice D) is a nutrient and energy source; it does not play a role in protecting the stomach and intestine from self-digestion.