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?

Answer: Option B

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.