Answer: The correct answer is C. the light.
Step-by-step explanation: A conditioned stimulus is a previously neutral stimulus, but thanks to being paired or associated with an unconditioned stimulus eventually elicit a conditioned response.
In this case, the light was a neutral stimulus but after pairing it repeatedly with the tickling of the nose it became a conditioned stimulus that elicits the conditioned response of sneezing.
The answer is C. The light is the conditioned stimulus (CS)