• The word "Loony" means crazy, irrational, mentally unstable, or foolish.
• We need to identify the word that has the opposite meaning.
• "Rational" means logical, sensible, and based on reason.
• Therefore, "Rational" is the correct antonym of "Loony."
• Hence, Option (a) "Rational" is the correct answer.

Additional Information

• Option (a) Rational — Correct. It means logical, sensible, and reasonable.
• Option (b) Strange — Incorrect. It means unusual or odd, which is similar in meaning to loony rather than its opposite.
• Option (c) Zany — Incorrect. It means amusingly crazy, eccentric, or unconventional, making it a synonym of loony.
• Option (d) Ridiculous — Incorrect. It means absurd or deserving ridicule, which is closer in meaning to loony than its antonym.