Suppose your alarm made a slight clicking sound just before the alarm goes off. Even though you didn't wake up to the clicking sound initially, now you do, due to classical conditioning. In this example, the clicking is a/an:

In this example, the clicking is a stimulus to wake you up.

Explanation:

The theory of classical conditioning works on the interplay between two entities or concepts which is the stimuli on the reaction it generates on the test subject or the person who is being assessed for classical conditioning

In the stimuli, the entity is the one that is causing the reaction to happens on one that gets associated with something else entirely different. Like, the clock's ticking alarm is not the sole reason for waking up but it certainly becomes the stimulus for it now.