Explanation:

- Vigorous: This word describes something full of energy, strength, and vitality, often associated with strong and forceful actions.

- Extreme (Option 1): This word refers to the farthest or most intense state of something, not directly opposite to vigorous.

- Delicate (Option 2): This word signifies something fragile, gentle, or easily broken, often lacking strength or robustness.

- Energetic (Option 3): This word is similar to vigorous, meaning full of energy and enthusiasm.

- Hard (Option 4): While it refers to being solid or firm, it doesn't directly oppose the concept of being vigorous.

The correct antonym for "vigorous" is delicate. The idea is that delicate implies fragility or weakness, contrasting the strength implied by vigorous.