Answer:
This best illustrates C. higher-order conditioning.
Explanation:
Higher-order conditioning is a term in Classical Conditioning in which a neutral stimulus (which does not create a response on its own) is paired with a conditioned stimulus (which creates a conditioned response) in order to be associated with the conditioned stimulus' conditioned response and thus create this response on its own.
In this case, the tone is the conditioned stimulus since it creates the conditioned response of salivation. The light is the neutral stimulus but once it is associated with the tone, it will also trigger the salivation.