Final answer:
The net external force on an object in free fall without air resistance is the force of gravity, which is downward with a magnitude of mg. Thus, the correct answer is F_net = mg.
Step-by-step explanation:
When an object falls through the air and air friction is neglected, the only force acting on it is the force of gravity. This net external force on the object is downward and has a magnitude equal to the product of mass (m) and acceleration due to gravity (g), which is represented by mg. Therefore, from Newton's second law of motion, Fnet = ma, and since the acceleration (a) is equal to the acceleration due to gravity (g), Fnet = mg. The option that matches this description is (c) Fnet = mg, indicating the net force is downward with a magnitude equal to the weight of the object.