Question

Sol. Key concept: Static friction supplies the torque needed for rolling.

Pedalling applies a driving torque to the rear wheel. Its point of contact tends to slip backward relative to the road, so the road exerts forward static friction on that wheel.

The front wheel is not directly driven. The axle pulls its centre forward, and backward static friction at the contact point provides the torque that increases its clockwise angular speed. Thus friction is backward on the front wheel and forward on the rear wheel: option (d).

Correction / clarification: For rolling without slipping, the contact point does not actually slide. The direction of friction opposes the tendency to slip, rather than an existing sliding motion.