Final answer:
The correct answer to the student's question is a. Methyl-morphine, morphine, 2D6, as codeine is known as methyl-morphine and is metabolized into the active metabolite morphine by the CYP 2D6 enzyme.
Step-by-step explanation:
Codeine, also known as methyl-morphine, is metabolized to the active metabolite, morphine, by CYP 2D6. Therefore, the correct answer is a. Methyl-morphine, morphine, 2D6. As a prodrug, codeine is transformed in the body into morphine, which then exerts the analgesic effects by acting on the central nervous system. This metabolism is primarily mediated by the cytochrome P450 enzyme CYP 2D6. Due to genetic variations in CYP 2D6, the effectiveness of codeine can vary significantly among individuals.