Final answer:
The "A" in the A-B-C-D-E mnemonic stands for Activating event. It represents the event or situation that triggers unproductive thinking or negative emotions.
Step-by-step explanation:
The "A" in the A-B-C-D-E mnemonic stands for Activating event. It represents the event or situation that triggers unproductive thinking or negative emotions. Identifying the activating event is the first step in the A-B-C-D-E model, which is used to identify and challenge unproductive thoughts and replace them with more rational and positive ones.