The correct answer is (c) hello.
Word Analysis:
— Adieu (French/English): a formal word meaning 'goodbye' or 'farewell'. Used when bidding someone a final or formal farewell.
Antonym: 'hello' — a greeting used when meeting someone; the opposite of farewell.
✗ farewell — synonym of adieu (both mean goodbye).
✗ good bye — synonym of adieu.
✗ valediction — the act of saying farewell; also a synonym.
Logic: Adieu = parting/departure word → Antonym = greeting/arrival word = 'hello'.
Source: Oxford English Dictionary; Merriam-Webster; BCS Previous Year Questions.