Pavlov, the researcher who studied classical conditioning, rang a bell every time he fed his dogs. At first the dogs drool in response to the food, but eventually began to drool in response to the bell alone (with or without the food). In this example, the food is the stimulus, and the bell is the stimulus.

a non-neutral or unconditioned stimulus

neutral stimulus

Explanation:

Ivan Pavlov developed a theory known as Classical conditioning. it is when two stimuli are linked together to produce a new learned response.

Ivan conducted a research with dogs. he demonstrated how the presence of food (stimulus) would lead to the dog salivating. salivation is known as an unconditioned response. When he gave the dogs food, he rang a bell. after a period, ringing the bell led the dogs to salivate. the bell is a neutral stimulus.

the answer to your question is,

d. when a child was born.

-mabel < 3