Complete the sentence with the correct form of the verb. Most of the Pharaohs' tombs ... (rob) by thieves.

The sentence describes a past event that happened to the tombs. The passive voice is required because the tombs were acted upon. The past perfect passive 'had been robbed' indicates an action completed before another past action or a specific time in the past, which fits the context of historical events. The simple past passive 'were robbed' is also a possibility, but 'had been robbed' often implies a sequence of events or a state resulting from past actions, which is common when discussing historical discoveries or conditions. Considering the context of historical tombs and their fate, the past perfect passive is the most appropriate choice to convey that the robbing occurred and was a completed event in the past, possibly before their discovery or before a certain point in history being discussed.

The correct answer is: had been robbed