Which of the following is an example of a conditioned reflex?

A conditioned reflex is a type of learned response that occurs as a result of the pairing of a neutral stimulus with a stimulus that naturally elicits a response. 

In the case of the hungry dog, the ringing of the bell (the neutral stimulus) is paired with the presentation of food (the natural stimulus) multiple times. Eventually, the sound of the bell alone is enough to elicit a salivation response in the dog, even in the absence of food. This is an example of classical conditioning, which was famously demonstrated by Pavlov in his experiments with dogs.

Withdrawing your hand from a pin prick is an innate reflex that occurs without any learning, as is the movement of digested food through the alimentary canal and the closing of your eyes in response to something entering them are examples of unconditioned reflexes.