The correct answer is $\boxed{\text{C}}$.
The tractive effort required to keep the truck moving at a uniform speed is equal to the product of the gross vehicle weight and the rolling resistance. In this case, the gross vehicle weight is 30 tonne and the rolling resistance is 30 kg/tonne, so the tractive effort is $30 \times 30 = 900$ kg.
Option A is incorrect because it is the rolling resistance per tonne, not the total tractive effort. Option B is incorrect because it is the rolling resistance of a single tonne, not the total tractive effort of a 30 tonne truck. Option D is incorrect because it is the total weight of the truck, not the tractive effort required to keep it moving.