If a person takes an aspirin and the aspirin makes a headache go away, then the person becomes more likely to take an aspirin for a headache in the future. This example illustrates that the aspirin acts as a(n):

negative reinforcer

Explanation:

In operant conditioning which involves learning being brought about by the consequence of a behavior, a negative reinforcer increases the likelihood of a behavior to be exhibited by the removal of an unpleasant or aversive stimulus.

Aspirin in the example given, is a negative reinforce that removes the headache, and as such increases the likelihood of the person taking aspirin in future when such aversive stimulus occurs.

i believe the answer would be negative peer pressure