Answer:
The trainer is using the method of A. successive approximations.
Explanation:
Successive approximations are steps taken in order to slowly achieve the desired behavior. As the name suggests, these steps are successive, meaning they start with little goals and when one is achieved it moves on to the next until the desired behavior is acquired.
This technique is generally used when training animals; as we can see in the example, the trainer want to teach the dog to balance on a ball. He/she does this by first making the dog approach the ball with a treat, then when placing its front paws on it, etc.
Little by little, the trainer moves forward from each step in order to achieve the desired behavior.