Final answer:
Niacin, or vitamin B3, can be synthesized in the body from tryptophan, which is an essential amino acid. This means the correct answer is a. niacin.
Step-by-step explanation:
The vitamin that the body can synthesize from the essential amino acid tryptophan is niacin (vitamin B3). Vitamins are organic compounds that are required in small quantities for bodily function, and they can be categorized into fat-soluble and water-soluble vitamins. While some amino acids can be synthesized by the body, others must be obtained from the diet. Niacin is unique in that it can be synthesized in the body from tryptophan, an essential amino acid. Specifically, it is estimated that 60 mg of tryptophan can give rise to 1 mg of niacin. Therefore, the correct answer to the student's question is a. niacin.