The answer is: After an accident with a red car last month, Giorgio gets nervous when he sees a red car, but not when he sees a red truck or van.
In classical conditioning, stimulus discrimination refers to the stimulus that create different response from you compared to other similar stimulus.
The discrimination could occurs if you experience positive things from the stimulus in the past (which make you addicted) or negative things from that stimulus (which made you traumatized). In Giorgio's case, the stimulus discrimination occurs because he experienced a traumatic experience with a red car, which influence his nervousness every time he see one.