Which process of behavior modification involves a person learning to associate a desired behavior with a previously unrelated stimulus?

The theory is called Conditioning. In behavioural psychologythe theory states that the reaction to an event by a person can be modified by conditioning a new response. So, the way they tested this theory was by ringing a bell every time this dog was about to get food. They got the dog conditioned so all it would need to hear was the bell and it would start drooling and carrying on as if it had actually seen or smelled the food.

The bell was an artificially injected stimulus in that bells don't normally ring for dog feeding time so there would be no natural connection for the dogs to have made instinctually to the sound of the bell and the food. By making and reinforcing this connection over and over they made it so the dogs stimulus created an unnatural reaction by the drooling at the sound of the bell.

The key here is that it is an unconditioned stimuli that creates the conditionoinged response.