Correct Answer: C

Rationale: The correct answer is C, leptin. Leptin is a hormone produced by fat cells that helps regulate energy balance by suppressing hunger, thus aiding in the regulation of body fat. Glucagon (choice A) is a hormone that raises blood glucose levels, ghrelin (choice B) stimulates appetite, and insulin (choice D) regulates blood sugar levels and promotes glucose uptake.

Correct Answer: B

Rationale: Iron deficiency is the most common nutrient deficiency globally. It leads to anemia and affects many populations, especially women and children. Magnesium, selenium, and zinc deficiencies are important, but they are not as prevalent as iron deficiency on a global scale.

Correct Answer: A

Rationale: The correct answer is A: Carbohydrates. Carbohydrates, especially fermentable sugars, are the most significant dietary factor in the development of dental caries. When these sugars are consumed, they can be fermented by bacteria in the mouth, leading to the production of acids. These acids lower the pH in the mouth, creating an acidic environment that promotes demineralization of the tooth enamel, ultimately causing dental caries. Choices B, C, and D are incorrect because while vitamins and minerals are important for overall health, they do not have the same direct impact on the development of dental caries as fermentable carbohydrates do. Fats, on the other hand, do not play a significant role in the development of dental caries.

Correct Answer: A

Rationale: The correct answer is A: Iron. Iron is an essential nutrient needed by the body to make hemoglobin, the protein in red blood cells that carries oxygen. While other nutrients like vitamin B12, niacin, and folate are important for various bodily functions, they are not directly involved in the production of hemoglobin. Vitamin B12 is essential for nerve function and DNA synthesis, niacin is important for metabolism, and folate is crucial for cell division and DNA synthesis. Therefore, only iron is specifically required for hemoglobin production.

Correct Answer: C

Rationale: The correct answer is C: 9. There are 9 essential amino acids that the body cannot synthesize and must be obtained through the diet. These 9 amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Choices A, B, and D are incorrect as they do not represent the correct number of essential amino acids.