The correct option is C exonerated
The word 'accused' refers to being charged with or on trial for crime.
Option A, liberated refers to being freed from enemy occupation.
Option B, impeached refers to charging with treason or other crime against the state.
Option C, exonerated refers to releasing someone from a duty or obligation.
Option D, sentenced refers to declaring the punishment decided for the offender.
Thus, according to the meaning, the word 'exonerated' is the opposite of the word 'accused'.