Which of the following terms refers to a lack of energy or vitality?

The term that signifies a lack of energy or vitality is indeed "sluggish." This word specifically describes a state of inactivity or lethargy, suggesting that an individual or object moves or responds slowly and lacks the usual vigor or liveliness.

In contrast, the other terms convey the opposite qualities. "Dynamic" refers to something characterized by constant change, activity, or progress, which inherently implies energy and vitality. "Vibrant" describes something full of energy, enthusiasm, or activity, often used to characterize lively colors or spirited people. "Energetic" directly refers to having or showing a lot of energy, making it clear that this term embodies vitality rather than a lack of it. Thus, "sluggish" is the only term among the options that appropriately aligns with the idea of a deficiency in energy or vitality.