A discriminative stimulus is:

1) a stimulus in the presence of which a response will be reinforced

2) a stimulus in the presence of which a response will not be reinforced

3) a stimulus in the presence of which a response will be placed on extinction

4) a stimulus that cues respondent behavior to occur

The correct answer is option 1. a stimulus in the presence of which a response will be reinforced

Explanation:

We call discriminative stimuli to those stimuli that indicate the probability that a given response is reinforced. The presence of a discriminative stimulus makes the appearance of the responses that have been reinforced in their presence more likely.

It's a neutral stimulus that indicates when the response may or may not lead to the desired consequence. That is, the discriminative stimulus indicates whether the enhancer is available.