Option 1 -> "secret" is a synonym of clandestine, not an antonym.
Option 2 -> "hidden" is also a synonym of clandestine, not an antonym.
Option 3 -> "overt" means open and public, which is the opposite of clandestine (secret/hidden).
Option 4 -> "congested" means crowded or blocked, unrelated to the meaning of clandestine.
Hence, Option 3: overt -> Clandestine means secretive or done in hiding, while overt means open, public, and observable. Since the meeting was clandestine (secret), the antonym would describe a meeting that is overt (publicly known) -> correct