The Cardassian leader who later fought for the Dominion was Gul Dukat.
Dukat began Star Trek: Deep Space Nine as the former overseer of Bajor and commander of Terok Nor, the station later renamed Deep Space Nine. After the Cardassian withdrawal, he was demoted from a senior command position but remained a powerful and ambitious political figure.
In the fifth-season episode “By Inferno’s Light,” Dukat is revealed to have secretly negotiated Cardassia’s entry into the Dominion. He became head of the Cardassian Union and helped lead the Dominion’s campaign, including the recapture of Deep Space Nine in “Call to Arms.” His alliance transformed him from recurring antagonist into one of the central political and military figures of the Dominion War.
The other options represent different Cardassian figures. Enabran Tain led the Obsidian Order, Gul Damar served as Dukat’s subordinate and later successor, and Gul Madred was an interrogator seen in The Next Generation.