Correct Answer: Delicate

Solution : The correct option is the second option.

Explanation:
Delicate is the best antonym here because it represents the opposite of vigorous, conveying something fragile or easily breakable, contrasting the robustness and strength associated with being vigorous.
The meanings of the other options are as follows:

Extreme means reaching the highest or farthest point.

Energetic means having or showing a lot of energy or enthusiasm.

Hard means difficult or firm.

Therefore, the correct answer is delicate.