Which word best describes a lack of energy?

The word "lethargic" is the most fitting choice to describe a lack of energy. It specifically refers to a state of sluggishness, inactivity, or lack of interest, where an individual feels tired and weak, displaying little to no enthusiasm for engaging in physical or mental activities.

In contrast, the other options convey different states of energy or activity. "Calm" suggests a state of tranquility or peacefulness, but it does not indicate a deficiency in energy. "Active" implies being engaged in physical movement or lively activity, which is the opposite of lacking energy. Similarly, "vigorous" describes a strong and energetic approach or activity, once again presenting the antithesis of lethargy. Thus, "lethargic" is the word that most accurately encapsulates the concept of lacking energy.