Which ancient structure was the largest gladiatorial arena?

The Colosseum, also known as the Flavian Amphitheatre, is recognized as the largest gladiatorial arena in ancient Rome. Completed in 80 AD, it could hold tens of thousands of spectators and was the site of numerous gladiatorial games and various public spectacles, including animal hunts and mock sea battles. The architectural design of the Colosseum, with its complex system of vaults and seating arrangements, allowed for a vast audience to observe the events unfolding in the arena below.

Other structures listed, such as the Markets of Trajan and the Theatre of Pompey, served different purposes and catered to different forms of entertainment, such as shopping and theatrical performances, rather than the specific combat-centric spectacles synonymous with gladiatorial arenas. The Circus Maximus, while also a large venue capable of accommodating many spectators, was primarily used for chariot races rather than gladiatorial events. Thus, the Colosseum stands out as the quintessential symbol of Roman gladiatorial combat, making it the correct answer.