When a penny is dropped from a height of 20 meters, the correct option is:
The force of gravity causes it to accelerate.
As the penny falls, it accelerates towards the ground due to the force of gravity, which acts on it constantly, resulting in an increase in its velocity until it reaches terminal velocity (if it were to fall for long enough in a real-world scenario). The other options provided are incorrect: the mass of the penny does not change, it does not change direction as it falls, and it does not fall at a constant velocity (it accelerates until air resistance brings a balance).