Final answer:
The boy's fear of cars and trains after being conditioned to fear a bus is an example of stimulus generalization. This learning process causes him to respond similarly to new stimuli that resemble the original conditioned stimulus.
Step-by-step explanation:
A boy who has been conditioned to be afraid of a bus and subsequently finds he is also afraid of cars and trains is experiencing stimulus generalization. This is a phenomenon in classical conditioning where an organism responds to new stimuli that are similar to the original conditioned stimulus with the conditioned response. In other words, after being conditioned to one stimulus, the boy's fear response has generalized to other, similar stimuli. This is the opposite of stimulus discrimination, where an organism learns to differentiate between various stimuli and responds accordingly. The generalization implies that the stimuli share common characteristics that trigger the same fearful response he has learned to associate with the bus.
In John B. Watson's experiments, the infamous case of Little Albert demonstrated stimulus generalization, where he was conditioned to fear a white rat and then generalized that fear to other white furry objects. The case of the boy fearing a bus and then similar vehicles such as cars and trains illustrates the same learning process.