Choose the option that is the correct passive form of the sentence.

We had eaten the cake.

The cake had been eaten by us.

The correct passive form of the sentence We had eaten the cake is The cake had been eaten by us.

The active voice of the sentence is We had eaten the cake. This sentence is in the past perfect tense, which is used to describe an action that had happened in the past before another action happened. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "we" and the main verb is "eat." The past participle of "eat" is "eaten." So, the passive form of the sentence is The cake had been eaten by us.

The other options are incorrect because:

• The cake will be eaten by us. is in the future tense, while the original sentence is in the past perfect tense.
• The cake is eaten by us. is in the present tense, while the original sentence is in the past perfect tense.
• The cake was eaten by us. is also in the past tense, but it is not the past perfect tense. The past perfect tense is used to describe an action that had happened in the past before another action happened. The action of eating the cake is not something that had happened before another action happened.