He tried to conceal the secret that they were already married.

Answer: Option C

Conceal : not allow to be seen; hide.
State : the particular condition that someone or something is in at a specific time.
Declare : say something in a solemn and emphatic manner.
Reveal : cause or allow (something) to be seen.
Announce : make a formal public statement about a fact, occurrence, or intention.

Antonym of Conceal is Reveal