Final answer:
The goldfish trained to select a blue disk associated with food under white light chose randomly under yellow light possibly because the change in lighting affected their ability to distinguish colors, disrupting their learned behavior.
Step-by-step explanation:
The most likely explanation for why goldfish chose the blue disk under white light but made random choices under yellow light pertains to visual perception and the impact of different light conditions on color visibility. Under white light, which includes the full spectrum of visible wavelengths, the fish could easily distinguish between the blue and gray disks due to their prior conditioning to associate the blue disk with food. However, under yellow light, which could potentially alter or wash out the blue colouration, the fish may have found it more difficult to differentiate the two disks, leading to random choices since neither contained food to guide their decision.