The thief confessed at the police station that he had stolen my watch.

Answer: Option C

Confessed : admit that one has committed a crime or done something wrong.
Concealed : kept secret; hidden.
Disproved : prove that (something) is false.
Denied : state that one refuses to admit the truth or existence of.
Admitted : confess to be true or to be the case.

Antonym of Confessed is Denied