A person is sitting in a car which is at rest. The reaction from the road at each of the four wheels of the car is R. When the car runs on a straight level road, how will the reaction at either of the front wheels vary?

When the car is at rest, the total weight of the car is evenly distributed among the four wheels.
So, each wheel experiences a reaction force of R.

When the car is moving, the engine provides a forward force.
Due to inertia, there is a tendency for the car's weight to shift towards the rear wheels.
Think about when you accelerate in a car; you feel pushed back into your seat.

This weight shift means that the rear wheels will bear a slightly larger portion of the car's weight.
Consequently, the front wheels will bear a slightly smaller portion of the car's weight.

Therefore, the reaction force at either of the front wheels will be less than R.
The correct answer is Option B.