Which word best conveys the idea of inactivity?

The term "quiescent" most accurately represents the idea of inactivity. It refers to a state of being dormant or at rest, emphasizing a lack of movement or activity. This word encapsulates the essence of stillness or idleness, making it suitable for describing something that is not currently active or engaged.

Other choices, while relevant in different contexts, do not convey the same concept of inactivity. "Reticent" describes someone who is reserved or reluctant to speak, which implies a lack of vocal activity but not a general sense of inactivity. "Repudiate" means to refuse to accept or be associated with something, indicating a rejection rather than stillness. "Soporific" refers to something that induces sleep or lethargy, suggesting a lack of activity but with an implication of drowsiness rather than a pure state of inactivity. Therefore, "quiescent" is the most fitting choice for conveying the idea of inactivity.