Alittle boy is allowed to play with a laboratory rat and shows no fear of it. then, a scientist makes a scary noise by banging an iron bar whenever the little boy reaches for the rat. soon, the boy begins crying whenever the rat comes near him. in this study, the scary noise is the

The scary noise in the scenario in which a little boy is allowed to play with a laboratory rat and shows no fear of it. then,and  a scientist makes a scary noise by banging an iron bar whenever the little boy reaches for the rat and because of this the boy begins crying whenever the rat comes near him is  unconditioned stimulus.
 Unconditioned stimulus is a stimulus that naturally, and automatically triggers a response. In this case such stimulus is the noise.