The cake _______ yesterday.

The correct answer is "was bought".

Explanation:

The sentence is in the Past Simple Passive voice. The passive voice is used when the subject (the cake) receives the action, rather than performs it. The word "yesterday" indicates that the action happened in the past.

Here's why the other options are incorrect:

• "will bought" is grammatically incorrect. "Will" requires the base form of the verb (be) followed by the past participle (bought) in the passive voice: "will be bought." It also refers to the future, which doesn't fit with "yesterday".
• "is bought" is in the Present Simple Passive voice. This would mean the cake is bought regularly, not just yesterday.

Therefore, the correct sentence is: "The cake was bought yesterday."