Alex Has Learned That If His Mom Is Upset and He

Alex has learned that if his mom is upset and he praises her, she will cook him a great meal. However, if he praises his mom when she is in a good mood, she responds by saying, "Are you trying to butter me up?" and doesn't cook at all. In this example, Alex's mom's mood is

A) a discriminative conditioned stimulus.
B) a reinforcer.
C) under stimulus control.
D) a stimulus predictor.