Answer:
A half second before the food
Step-by-step explanation:
According to the Pavlov's dog study, a conditioned reflex can be taught to a subject through presenting it in an appropriate manner. This process of presenting the stimulus at certain conditions or scenarios is known as conditioning. Here, the process of conditioning can be achieved if the dog is presented with light immediately before the food is presented. This way the dog will salivate to the light thinking it is a signal of getting food. Hence, the correct answer is that the light should be presented immediately before or a half second before the food so that the dog is conditioned to salivate to a light.