The Thebans were the Greek contingent at Thermopylae that eventually surrendered to the Persians. About 400 Thebans were present during the final stage of the battle in 480 BC.
Their role is more complicated than the usual brief quiz answer suggests. Herodotus says the Thebans were kept with the Greek force as hostages because Thebes was suspected of supporting Persia. Later historians have suggested they may instead have been Theban loyalists who opposed Persian domination and could not safely return home.
When the final Greek position was surrounded, the Thebans moved toward the Persians with raised hands and surrendered. Xerxes reportedly had the surviving Thebans branded with the royal mark. The Spartans and Thespians, by contrast, remained to fight until they were killed.
The common mistake is to confuse the Thebans with the Thespians. Both contingents were from Boeotia and fought at Thermopylae, but the 700 Thespians chose to remain and die in the final stand, while the Thebans surrendered.