Help the frustrated roommate complete their dramatic text message.

When I confronted him about the missing chocolate cake, he insisted that he ___ it. However, the frosting on his nose told a different story!

The correct answer is hadn't eaten.

The reporting verb "insisted" is in the past tense. The original statement ("I didn't eat it") was in the past simple. In formal reported speech, a past simple action that happened before the reporting verb is backshifted to the past perfect ("hadn't eaten").