The situation is an example of negative reinforcement.
In the condition of negative reinforcement, a stimulus especially an unpleasant stimulus is removed due to a particular behavior of the person. In this case, the unpleasant behavior is the headache, and taking aspirin is a behavior of Nancy, which cut of the stimuli, that is pain relieving. In negative reinforcement a particular behavior is increased, in order to remove the unpleasant stimulus.
