The correct answer is D) rotor clearances. In a gear type rotary pump, the capacity or flow rate of the pump is directly related to the clearances between the rotating gears or rotors and the pump housing. As these clearances increase due to wear or other factors, the pump's ability to efficiently move fluid from the suction side to the discharge side decreases, resulting in a lower overall flow rate or capacity. The other answer choices are incorrect because they do not directly impact the pump's capacity in the same way as increasing rotor clearances. Suction pressure, discharge volume, and torque rating are not the primary factors that determine the volumetric efficiency and capacity of a gear type rotary pump when operated at a constant speed.