Answer:
The stimulus that acts as an Unconditioned stimulus.
Step-by-step explanation:
A discriminative stimulus is the stimulus that has control over an specific behavior because the behavior was reinforced in the presence of that stimulus in the past. This response takes place after learning and it is this stimulus the one which will be paired with the conditioned stimulus to create the conditioned response.
a) A stop sign: this stimulus automatically triggers the response of stopping when we see it, so there is not really a reinforcement caused by this stimulus.
b) The white rat in Little Albert studying of producing phobias: at first Little Albert wasn't afraid of the white rat. It was after it was associated with the loud noise (discriminative stimulus) that he became afraid of it.
c) The stimulus that acts as a UCS: The unconditioned stimulus (UCS) represents something that naturally and automatically triggers a response. So it can be an example of discriminative stimulus since it will be the one that will act as a reinforcer of a conduct in presence of this stimulus.
d) A salivating dog in Pavlov's laboratory: This is not a stimulus but rather a behavior.
Thus, the correct answer is c)