Final answer:
The stroking of the side of the infant's head has become associated with the act of nursing, allowing it to elicit a calming response when the infant is upset.
Step-by-step explanation:
The correct answer is d. conditioned stimulus.
In classical conditioning, a conditioned stimulus (CS) is a previously neutral stimulus that, through repeated pairings with an unconditioned stimulus (UCS), acquires the ability to elicit a conditioned response (CR). In this case, the stroking of the side of the infant's head has become associated with the act of nursing, which is the unconditioned stimulus. As a result, the stroking of the side of the head now has the ability to calm the infant down when they are upset, which is the conditioned response.
This can be observed as a learned response that has been acquired through repeated pairings of the conditioned stimulus (stroking the side of the head) with the unconditioned stimulus (nursing).