English Antonyms Practice Test 8

Obligatory : required by a legal, moral, or other rule; compulsory.
Optional : available to be chosen but not obligatory.
Superfluous : unnecessary, especially through being more than enough.
Necessary : needed to be done, achieved, or present; essential.
Advisable : (of a course of action) to be recommended; sensible.

Antonym of Obligatory is Optional