Final answer:
The forces acting on a man falling at constant velocity with an open parachute are gravity and air resistance, which are equal and opposite, resulting in zero net force and maintaining terminal velocity.
Step-by-step explanation:
When a man jumps from an airplane and opens his parachute, falling at constant velocity, the forces acting upon him are the force of gravity pulling him downwards and the opposing force of air resistance acting in the upward direction. According to Newton's Second Law, at constant velocity, the total net force acting on the man is zero, which indicates that the magnitude of these two forces is equal. The correct answer to the question is C. The same two forces act on him as before; the only change is that, with the parachute open, the two forces sum to zero.
Gravity continues to pull the man towards the Earth with the same force as when he was freely falling without the parachute. When he opens his parachute, the cross-sectional area of the man-parachute system increases drastically, leading to a significant increase in air resistance (or drag force). This additional air resistance is what slows the man down and eventually balances the gravitational force, thus achieving a state of no acceleration, known as terminal velocity.