Gary trains his hamster to roll a marble. first, he reinforces the hamster when it walks toward the marble, then he reinforces when it touches the marble with its nose. finally, the hamster learns to roll the marble and is reinforced for it. this example illustrates

The correct answer is shaping.

Shaping refers to a behavioral learning process whereby individuals or animals are gradually taught or trained how to respond to or behave, through reinforcement of responses that are similar or close to the target or desired behavior/ response. In Gary's instance, by rewarding his hamster when it touches a marble with its nose, he is teaching the hamster to gradually roll the marble for a reward. 

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