Stuart Is Afraid of Public Speaking, but Only When He

Stuart is afraid of public speaking, but only when he has to make speeches on the weekend. He is a professor and has no trouble speaking in front of large groups of students, and he has made effective presentations at conferences, as long as he presents on a weekday. The specificity of Stuart's fear mostly clearly demonstrates the process of:

A) negative reinforcement.
B) discrimination.
C) avoidance conditioning.
D) stimulus generalization.