The correct option is A compulsory
The word 'voluntary' refers to acting according to one's own free will.
Option A, compulsory refers to something obligatory or mandatory.
Option B, violent refers to something involving physical force intended to hurt or damage to someone.
Option C, deliberate refers to something being done consciously and intentionally.
Option D, valuable refers to something worth a great deal of money.
Thus, according to the meaning, the word 'compulsory' is the opposite of the word 'voluntary'.