Concept:The correct option must be the closest opposite of the underlined word and also fit grammatically in the gap.
Explanation:The underlined word is
compulsory, which means required by authority, rule, or force.
When something is compulsory, a person has no choice but to do it.
The direct opposite of this idea is
voluntary, which means acting from one's own free will or choice without external pressure.
Among the given options, only "voluntary" carries this meaning.
"Wilful" means done intentionally or stubbornly, not because of obligation.
"Deliberate" means planned or done on purpose, not opposite of being forced.
"Outright" means complete or total, which is also unrelated to compulsion.
Therefore, the sentence is best completed by placing "voluntary" in the gap because it is the most nearly opposite in meaning to the underlined word.
Answer:B. voluntary