Correct Answer & Explanation
Option C
• 'Feeble' means lacking physical strength or vigor, especially as a result of age, illness, or weakness.
• 'Frail' means weak and delicate, which is the exact synonym of 'feeble'.
• Options A, B, and D are antonyms: 'Robust', 'Potent', and 'Energetic' all describe strength, power, and vitality.