I believe the answer is: A. conditioned stimulus
The conversion is being done by pairing the neutral stimulus with a specific outcome. In pavlov experiment, this being done by blowing a whistle before giving the dogs their food.
In the beginning, the whistle is a neutral stimulus that does not give any effecy to the dog. But overtime, hearing the whistle make the dog automatically salivated. When this happen, the neutral stimulus has become a conditioned stimulus.
