The correct answer is B. 30 kg/tonne.
The grade resistance factor is the force per tonne of weight that is required to overcome the resistance of the road surface. The tension in the tow cable is the total force required to pull the truck up the hill. The rolling resistance of the road is the force per tonne of weight that is required to overcome the friction between the tires and the road surface.
To calculate the rolling resistance of the road, we can use the following equation:
Rolling resistance = Tension in the tow cable / (Operating weight / 1000)
= 720 / (12000 / 1000)
= 30 kg/tonne
Therefore, the rolling resistance of the road is 30 kg/tonne.
Option A is incorrect because it is the grade resistance factor. The grade resistance factor is the force per tonne of weight that is required to overcome the resistance of the road surface.
Option C is incorrect because it is the tension in the tow cable. The tension in the tow cable is the total force required to pull the truck up the hill.
Option D is incorrect because it is the sum of the grade resistance factor and the rolling resistance of the road. The grade resistance factor and the rolling resistance of the road are two different forces.