What is the main factor that determines terminal velocity?

Terminal velocity is the constant speed that an object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration. The primary factor that determines terminal velocity is the balance between the downward force of gravity and the upward force of air resistance. When these two forces are equal, the net force acting on the object is zero, causing it to fall at a constant speed—this is the terminal velocity.

The mass of the object plays a role in determining the force of gravity acting on it, while the surface area affects the amount of air resistance encountered. However, it is the point at which the forces of air resistance and gravitational force are equal that defines terminal velocity. Thus, the correct answer highlights the equilibrium of these two forces that leads to the condition of terminal velocity.