Answer:
Jonathan buckling the seat belt
Step-by-step explanation:
The conditioned stimulus in the present situation is Jonathan buckling the seat belt
A conditioned stimulus is generally considered as a neutral stimulus. It invokes to the mind automatically as a result of practice and training.
Here Jonathan always buckles the seat belt as soon as he sits in the car before starting the car. This habit is a result of a previous stimulus that the car buzzer goes off every time he starts the car before buckling the seat belt.
Thus the answer is
Jonathan buckling the seat belt