Final answer:
The heart (D) is not a primary site of drug metabolism; this role is primarily taken by the liver with other organs playing subsidiary roles in the breakdown or excretion of drug metabolites.
Step-by-step explanation:
The correct answer is D. Heart. The primary site for drug metabolism in the body is the liver, where various substances are broken down or modified. The kidneys play a role mainly in excretion of metabolites rather than metabolism, plasma may contain enzymes that can metabolize some drugs, and the lungs can metabolize certain volatile compounds. However, the heart is not considered a major site for drug metabolism. It's more involved in the circulation of blood than in drug metabolism.