Correct Answer: D

Rationale: The correct answer is D: By enterokinase. Trypsin is activated by enterokinase, an enzyme secreted by the small intestine. Enterokinase plays a crucial role in converting trypsinogen into its active form, trypsin. Choices A, B, and C are incorrect because trypsin is specifically activated by enterokinase, not by zymogen, bile, or gastrin.

Correct Answer: D

Rationale: The correct answer is D, eggs. Eggs are considered a high-quality protein source as they are rich in indispensable amino acids. Lentils, tofu, and peanuts also contain protein but are not as high in indispensable amino acids as eggs, making them less complete protein sources.

Correct Answer: B

Rationale: Hemoglobin is a protein that carries oxygen in the blood, making it the correct answer. Choice A, Saline, is a salt solution and not a protein. Choice C, Cellulose, is a complex carbohydrate found in the cell walls of plants, not a protein. Choice D, Prostaglandins, are lipid compounds with hormone-like effects, not proteins.

Correct Answer: A

Rationale: The correct answer is A: 9. Nine amino acids are considered essential or indispensable because they cannot be synthesized by the body and must be obtained through the diet. These amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Choices B, C, and D are incorrect as the number of essential amino acids is specifically 9, as they are crucial for various bodily functions.

Correct Answer: C

Rationale: Chymotrypsin is an enzyme found in pancreatic secretions that plays a role in protein digestion. Rennin is primarily found in the stomach and is involved in milk digestion. Pepsin is also a stomach enzyme that aids in protein breakdown. Casein is a protein found in milk, not an enzyme involved in pancreatic secretions. Therefore, the correct answer is Chymotrypsin.