Final answer:
The smallest radius of an unbanked track is equal to the radius of the vehicle's wheels.
Step-by-step explanation:
The smallest radius of an unbanked (flat) track is B) Equal to the radius of the vehicle's wheels.
When a vehicle moves on an unbanked track, the maximum friction between the tires and the track determines the maximum centripetal force that can be exerted. This force is given by the product of the coefficient of friction and the weight of the vehicle. The maximum centripetal force required to keep the vehicle moving in a circular path is determined by the vehicle's speed and the radius of the turn.
Therefore, in order for the vehicle to negotiate the curve without slipping, the radius of the unbanked track must be equal to the radius of the vehicle's wheels.