Final answer:
In classical conditioning, the nurse's phrase and the painful needle serve as the conditioned stimulus and unconditioned stimulus, respectively.
Step-by-step explanation:
In this example of classical conditioning, the UCS (unconditioned stimulus) is the painful needle, the CS (conditioned stimulus) is the nurse saying 'This won't hurt a bit', the UCR (unconditioned response) is the fear and pain felt when stabbed with the needle, and the CR (conditioned response) is cringing in fear when hearing the phrase 'This won't hurt a bit'.