Final answer:
To make the net torque on the wheel zero, a third force must be applied that counterbalances the existing torques. Since the forces already present are of equal magnitude but opposing directions, the third force must be a diagonal force, to provide the necessary counteracting torque.
Step-by-step explanation:
To determine which third force on the wheel, applied at point P, would make the net torque zero, we need to take into account the direction and magnitude of the current forces acting on the wheel. Vector C has the same length as the horizontal force, indicating that these two forces are equal in magnitude. Since torque is dependent on the force applied, the distance from the pivot point, and the angle of application, to make the net torque zero, the third force must be applied in such a way that its torque counterbalances the existing torques on the wheel.
If we already have a force causing a clockwise rotation and another causing a counterclockwise rotation, the third added force must produce torque that will counter the existing torque. If vector C points upwards and there's a horizontal force of equal magnitude, a diagonal force will likely produce the necessary torque that counteracts the other two, provided it's applied at the correct angle. It is not just the direction of the force but also the location and angle at which it is applied that matters for creating torque.