Select the most appropriate ANTONYM of the given word.

OPTIONAL

Compulsory

The correct answer is option 4. Compulsory.

"Optional" refers to something that is not required or mandatory; it's up to the individual's discretion whether to choose it or not.

The antonym we're looking for should describe something that is required or mandatory, something that one must do or participate in.

"Compulsory" fits this description perfectly. It means required by rule, obligation, or law; obligatory.

Therefore, "compulsory" is the appropriate antonym for "optional" because it represents the opposite characteristic—something that must be done or is required instead of being left to choice.