Many a man ___ killed by them.

The phrase “Many a/an” is a grammatical construction that, despite implying plurality, always takes a singular noun and a singular verb. It emphasizes individual items within a large group.

• Was is the correct option because the subject “Many a man” requires a singular verb. Although it refers to multiple men, the structure treats it as a singular unit for verb agreement.
• Were is incorrect because it is a plural verb, used with plural subjects.
• Had been is incorrect because while it is singular, it implies a past perfect tense, suggesting an action completed before another past action, which is not the simple past tense required here.

The sentence describes a simple past event affecting many individuals.