Selena once received a shock while plugging in her toaster and is now quite fearful each time she uses it. in this example, the toaster would be a(n) stimulus with respect to fear.

The correct answer is a conditioned stimulus.

According to the theory of classical conditioning, a conditioned stimulus (CS) refers to a previously neutral stimulus, that when associated with an unconditioned stimulus (UCS) is sufficient to elicit a response in the absence of the UCS. In Selena's instance, the CS is the toaster, whereas as the UCS is the electric shock. After the CS (plugging in of the toaster) was followed by the UCS (an electrc shock), the CS alone led to a response (fear when using the toaster) in Selena.