In Watson and Rayner's "Little Albert" study, each time the rat was presented to the boy it was accompanied by a loud noise. In this experiment, Albert's reaction of fear upon hearing the loud noise was the

unconditioned response.

Explanation:

Based on the information provided within the question it can be said that Albert's reaction would be considered as the unconditioned response. This term refers to a natural reaction to a specific stimulus, which has not been learned. Such responses include fear, happiness, sadness etc. Which in this case Albert is experiencing fear due to the loud noise which would be the unconditioned stimulus