Any stimulus an animal or person will avoid, given the opportunity to do so is:
a) an aversive
b) an avoidant stimulus
c) a reinforcer
d) a punisher

Any stimulus an animal or person will avoid, given the opportunity to do so is: an aversive

Explanation:

Aversives are abhorrent stimuli which cause variations in behavior through punishment; by implementing an aversive promptly following a behavior, the reasonableness of the behavior happening in the future is decreased. Aversives can differ from being lightly unpleasant or irritating to physically abusing.

It is not the level of unpleasantness, but preferably the effectiveness the unpleasant event has on modifying behavior that determines the aversive. It's more generally used in very precise writing, though, especially on the topics of psychology and sociology. The application of aversive methods and punishment procedures, is one of the main ideas that the public requires security by licensing behavior analysis.