Question

stimulus or event that increases the probability that the response it follows will be repeated
percept
melatonin
reinforcer
stressor
stimuli

Explanation:

In psychology, a reinforcer is a stimulus or event that increases the likelihood of a response being repeated. A percept is related to perception, melatonin is a hormone, a stressor causes stress, and stimuli are general input - not specifically related to response repetition increase.

Answer:

C. reinforcer