Final answer:
In the Little Albert experiment, the loud sound was the unconditioned stimulus which, when paired with the white rat, led to Little Albert's conditioned fear response to the rat and other furry objects.
Step-by-step explanation:
In the study with Little Albert, the unconditioned stimulus (UCS) is the loud sound that was paired with the white rat, which before the conditioning did not elicit fear from Little Albert. The fear that Albert showed was the unconditioned response (UCR) to the loud sound. After repeated pairings of the rat (neutral stimulus turning into conditioned stimulus) with the loud sound (unconditioned stimulus), Albert began to show the fear response (now a conditioned response) to the rat alone without the loud noise needing to be presented.
Regarding the options provided in the student question, the correct answer is 'a) the loud sound.' After conditioning took place, Little Albert also demonstrated stimulus generalization, as he started fearing not just the white rat but also other furry objects, including a rabbit, a furry coat, and even a Santa Claus mask.