The correct answer is C. compulsory.
A voluntary action is one that is done by choice, while a compulsory action is one that is required or enforced. Ordered, alternative, and essential are all words that can describe actions that are not voluntary, but they do not mean the same thing as compulsory.
An ordered action is one that is given as a command or instruction. For example, a police officer might order a suspect to put their hands up. An alternative action is one that is possible but not required. For example, you might have the alternative of taking the bus or walking to work. An essential action is one
that is necessary or very important. For example, it is essential to eat food to survive.In conclusion, the only word that is an antonym of voluntary is compulsory.