Zack imitates his trainer's stance when he performs a weightlifting exercise. in this instance, zack is demonstrating:

In this instance, Zack is demonstrating observational learning.

He is observing his trainer, watching him perform an exercise in order to see what he is supposed to do. He is learning by watching his trainer perform, and then he can replicate what the trainer has done. By imitating the exercise perfectly he will show that he has mastered the learning.