If a child gets upset and cries after receiving an injection at the doctor's office and now cries when he is told they need to visit the doctor, the mention of the doctor's office is a/an

The answer is conditioned stimulus. It was before a neutral stimulus that, after becoming linked with the unconditioned stimulus, it then becomes to cause an a conditioned response to happen. In this problem, the conditioned stimulus would be the mentioning of the doctor's since you remember that you've cried because you got shots. So every time you hear this, then you will cry.