How is a discriminative stimulus (SD) best defined?

A discriminative stimulus (SD) is best defined as a cue that signals the availability of reinforcement. This means that the presence of an SD indicates to the individual that a particular behavior, if performed, has a higher likelihood of being followed by reinforcement. In behavior analysis, understanding the role of discriminative stimuli is crucial because they help shape behavior by providing context for when certain responses will be reinforced.

For instance, if a child learns that saying "please" when asking for something usually results in receiving that item, "saying please" becomes associated with the SD, suggesting that reinforcement (the item they desire) will be available if the behavior is performed. This relationship guides future behavior, as the presence of the SD influences the likelihood that the behavior will occur.

The other choices do not capture the essence of what an SD represents. A response that results in reinforcement is more about the relationship between behavior and outcomes rather than the signal that cues a behavior. A behavior that is usually not reinforced does not describe an SD, as it does not fulfill the role of providing information about the availability of reinforcement. Lastly, a variable that influences random assignment does not pertain to the concept of discriminative stimuli and is unrelated to the signaling aspect of reinforcement availability.