At the End of the Day, Isaac Hears a Car

At the end of the day, Isaac hears a car pull in the driveway. A few minutes later, his father walks through the side door to the house. Isaac smiles, then runs to the door to greet his father. This scene is repeated for several days. Then, one evening, Isaac runs to the side door as soon as he hears a car pull into the driveway. Isaac's behavior of running to the door when he hears the car can best be described as which of the following?

A) Conditioned stimulus
B) Conditioned response
C) Unconditioned stimulus
D) Unconditioned response