The correct answer is A. Reduce by 200 kg.
The drawbar pull of a tractor is the force that it can exert on a trailer or other load. It is equal to the weight of the tractor minus the rolling resistance of the road. The rolling resistance is the force that opposes the motion of the tractor due to the friction between the tires and the road.
In this case, the tractor has a weight of 20 tonnes and a drawbar pull of 2500 kg. The rolling resistance of the road is 30 kg per tonne, so the total rolling resistance is 600 kg. This means that the net force that the tractor can exert on a trailer is 2500 – 600 = 1900 kg.
If the rolling resistance of the road is increased to 40 kg per tonne, the total rolling resistance will be 800 kg. This means that the net force that the tractor can exert on a trailer will be 2500 – 800 = 1700 kg.
Therefore, the drawbar pull of the tractor will reduce by 200 kg when the rolling resistance of the road is increased from 30 kg per tonne to 40 kg per tonne.
Option B is incorrect because the drawbar pull of the tractor will decrease, not increase.
Option C is incorrect because the drawbar pull of the tractor will decrease by 200 kg, not 250 kg.
Option D is incorrect because the drawbar pull of the tractor will decrease, not increase.