Final answer:
In John Watson's 'Little Albert' experiment, the UCS was d. the loud noise.
Step-by-step explanation:
In John Watson's 'Little Albert' experiment, the UCS (Unconditioned Stimulus) was the loud noise (D).
The experiment involved pairing the loud noise with a white rat, ultimately leading to Little Albert fearing the white rat alone. The loud noise served as the UCS because it elicited a reflexive fear response from Little Albert.