Final answer:
In psychology, a speaker's command acting as a discriminative stimulus for the listener's response is considered A. true. A discriminative stimulus signals potential reinforcement for a specific behavior.
Step-by-step explanation:
The question refers to a concept from psychology, particularly from the area of behavior analysis or operant conditioning. In such contexts, 'SD' stands for discriminative stimulus. A discriminative stimulus is an antecedent stimulus that signals the availability of reinforcement for a particular response. Therefore, if we consider that a "speaker's mand" (which likely refers to a command or instruction given by the speaker) is a signal that prompts the listener to act in a certain way, anticipating a potential reinforcement, it can be deemed a discriminative stimulus. So, the statement 'A speaker's mand is an SD for the listener to act' would be considered A. True.