Final answer:
The term that would best mean an agent that promotes or brings about sleep is 'C. Hypnotic' which directly refers to an agent or a substance that induces sleep.
Step-by-step explanation:
If HYPN- means sleep, the term that could best mean an agent that promotes or brings about sleep is C. Hypnotic. The prefix "hypn-" signifies a relation to sleep, and the suffix "-otic" implies something that pertains to, or is connected with. Consequently, the term "hypnotic" refers to an agent or a substance that induces sleep or a state resembling sleep closely. On the other hand, "hypnagogic" refers to the state immediately before falling asleep, and "hypnopompic" pertains to the state immediately upon waking. "Hypnotherapy" is a form of therapy in which hypnosis is utilized to create subconscious change in the form of new responses, thoughts, attitudes, behaviors, or feelings.