Bjorn's alarm clock makes a faint clicking noise a second before the alarm goes off. At first, the clicking noise doesn't wake Bjorn, but the alarm does. After a couple weeks, he awakens as soon as he hears the clicking noise. The clicking noise is a (n)

unconditioned stimulus

Explanation:

Unconditioned stimulus -

It is the type of stimulus which automatically, unconditionally triggers any response, is referred to as unconditioned stimulus.

The example of an unconditioned stimulus, is the smell of very delicious food, automatically makes a person hungry.

Hence, from the question, as the alarm clock act as a unconditioned stimulus.