Help the weary detective complete his dramatic memoir by choosing the grammatically correct phrase.

Over my thirty years on the police force, _____ attempted to outsmart the legendary Inspector Whiskers, only to fail miserably.

The correct answer is many a criminal mastermind has.

The formal determiner phrase "many a" is used to refer to a large number of people or things, but it strictly requires a singular countable noun and a singular verb (e.g., "many a mastermind has").