I wish I __________ more time to prepare for the exam.

The correct answer is "had". This sentence expresses a wish about a present or future situation that is contrary to reality or currently unfulfilled. When we use the phrase "I wish" to talk about a present or future situation we desire to be different from what it actually is, we typically follow it with the past simple tense. This grammatical structure is a form of the subjunctive mood, which is used to indicate that the wish is for something that is not currently true or is unlikely to happen. For instance, "I wish I had more time" clearly conveys the speaker's regret or desire for a different present reality, implying "I don't have more time, and I regret this." It's important to distinguish this from wishes about past events, which would require the past perfect tense (e.g., "I wish I had had more time yesterday"). However, since the wish here pertains to a present lack of time, the simple past "had" is the grammatically correct and appropriate choice. This construction effectively communicates the speaker's longing for a different current circumstance.