A: A discriminative stimulus is an element; be that a drug, or presence of a particular food or being that produces reinforcement, stimulation or positive reward. In studies, rats are trained to discriminate, or identify and differentiate a drug and particularly, a drug which they know from prior experience, will produce a positive effect or rewarding effect, thus, a discriminative stimulus simply means a substance which acts as a stimulus, and that the animal can differentiate what that substance is.
In humans, this occurs much more intentionally, and chronically, and under (often) self-made conditions. Meaning, a human discriminates that cocaine will produce a 'high', the cocaine is the stimulus that is being recognized, thus, it is discriminated, and thus, is a discriminative stimulus.
Much the same pattern goes for rats and other lab animals, just that the reasons for doing so are based on the initiative of the overseeing entity (the human, or tester), therefore the rats aren't taking in a drug to get 'high', but because it motivates them or produces a shift in their mood. If the human, or tester wasn't there, the animal would never know it existed.