The correct answer is: B. 6 kg/tonne
The grade resistance factor is a measure of the amount of force required to move an earth moving machine up a slope. It is calculated by multiplying the grade percentage by a factor of 6 kg/tonne.
For example, if a machine is operating on a slope of 10%, the grade resistance factor would be 60 kg/tonne. This means that the machine would need to exert a force of 60 kg for every tonne of its own weight in order to move up the slope.
The grade resistance factor is an important factor to consider when planning earth moving operations. It can affect the choice of machine, the speed of operation, and the amount of fuel required.
The other options are incorrect because they do not represent the correct factor for calculating the grade resistance factor.