A child starts fearing after seeing a black cow, black dog and black objects.…

Concept: In classical conditioning, once a response (such as fear) becomes conditioned to a specific stimulus, that same response can also be triggered by other stimuli that resemble the original one in appearance. This spread of one response to similar-looking stimuli is called stimulus generalization -- the more physically similar a new stimulus is to the original conditioned stimulus, the stronger the generalized reaction.

Application: Here, the child's conditioned fear was first attached to the black cow (the original conditioned stimulus). Black dogs and other black objects share the key perceptual feature -- the colour black -- with the cow, so the same fear response now extends to them as well. This is a textbook case of stimulus generalization: one response spreading across stimuli that resemble each other.

Cross-check: If the child had instead shown several different reactions (say, crying, freezing, and running away) specifically to the cow alone, that would point to response generalization, not stimulus generalization -- response generalization is about one stimulus producing a variety of responses, while here it is the reverse: one response (fear) recurring for a variety of similar-looking stimuli.

So the term that correctly names one conditioned response extending to new stimuli that resemble the original one is stimulus generalization.