c. grilled onions
Initially a neutral stimulus does not cause a response. However, after it is conditioned it becomes something that can produce a response. At first, grilled onions have no effect on Francis. Since Carla often smells of grilled onions when she sees Francis, Francis begins to associate the smell of grilled onions with Carla. It acts as a neutral stimulus that reminds Francis of Carla and creates the desire for him to see her.