He pleads for sanity in a ____ world.

The correct answer is Lunatic because the sentence structure demands an adjective that forms a stark, logical contrast with the concept of 'sanity'.

• Lunatic: This is the correct choice. To 'plead for sanity' means to ask for sensible or rational behavior. This action becomes contextually meaningful when the surrounding environment is irrational, foolish, or wild ('a lunatic world').
• Incorrect Options:
  • Reasonable / Cautious: These are incorrect. Pleading for sanity inside a 'reasonable' or 'cautious' world would be completely redundant, as those environments already embody sane characteristics.