Teaching students how to play tennis, an instructor decides to break down the skill into its component parts and teach each in turn. this approach is an example of

The answer is . shaping.
In shaping, we aim to condition the desired behavior by giving the subject a reward gradually after they can finish several parts of the task.
In tennis, this could be done by separating the training, starting from the easiest technique to the hardest technique, in order to make the trainee felt certain sense of accomplishment/improvement.