The correct option is C reveal
Conceal means prevent (something) from being known; keep secret. The opposite of conceal is reveal, or make (previously unknown or secret information) known to others. Hence, Option C is correct. Among the rest of the options, state means express something definitely or clearly in speech or writing, declare means say something in a solemn and emphatic manner, announce means make a formal public statement about a fact, occurrence, or intention. These options do not express this antonym, hence incorrect.