Final answer:
AID converts cytosine into uracil, not into adenine, thymine, or guanine. Uracil is typically found in RNA, while thymine occurs in DNA.
Step-by-step explanation:
AID (Activation-induced cytidine deaminase) is an enzyme that changes the nitrogenous base cytosine into uracil, not into adenine, thymine, or guanine. In DNA, uracil is not normally present since it is the base that is substituted by thymine. However, during the process of cytidine deamination, cytosine is converted to uracil, which can then be recognized and replaced by the appropriate DNA repair mechanisms. This process is crucial for somatic hypermutation and class switch recombination within the immune response. The answer to the student's question, therefore, is C. Uracil.