Why did you help him when you knew that he . . . . . . . . you?

The correct answer is C. cheated.

The other options are not correct because they do not describe a past action.

• Option A, “will cheat,” is not correct because it describes a future action.
• Option B, “is cheating,” is not correct because it describes an ongoing action.
• Option D, “had cheated,” is not correct because it describes a past action that is no longer relevant.

The correct answer, “cheated,” describes a past action that is relevant to the question because it explains why the speaker helped the person even though they knew that they had cheated.