Adam drops a marble and a penny from the top of a 2-meter-tall vacuum, a space free of air particles. He observes that both objects land at the same time. This is example of which scientific law?

On the moon, both will land at the same time. The marble resistance is comparable to it's gravitational acceleration at very fast speed (terminal velocity), so it's free fall acceleration is halted fairly slowly where as the coin is able to accelerate at the same rate (9.8 m/s^2) to a higher speed before air resistance cancels out gravity.