Final answer:
A client prescribed nifedipine, a medication for high blood pressure and angina, should avoid grapefruit juice as it can interfere with the medication's metabolism.
Step-by-step explanation:
The client with a new prescription for nifedipine, which is a calcium channel blocker used for managing high blood pressure and angina, should be advised to avoid grapefruit juice. Grapefruit juice can interfere with the metabolism of various medications, including some calcium channel blockers, leading to increased blood levels of the medication which can enhance the effects and side effects.
There is no significant interaction between nifedipine and milk, aged cheese, or bananas that would necessitate avoiding these foods. Therefore, the correct food for the client to avoid is C.) Grapefruit juice.