Correct Answer: lethargic

Solution : The third option is the correct choice.

Agile means to be quick and well-coordinated in movement. Lethargic means having little energy and feeling unwilling and unable to do anything. It is clear that "agile" and "lethargic" have opposite meanings.

The meanings of other words are as follows:

• Youthful means to be young or have characteristics of young people.
• Prompt means to make someone decide to do something.
• Energetic means showing or involving great activity or vitality.

Therefore, the correct answer is lethargic .