To increase the volumetric output of a fixed displacement pump, you should?

Increasing the volumetric output of a fixed displacement pump involves making adjustments that enhance its performance under the given conditions. Steering the focus towards the chosen option of speeding up the prime mover is key because it directly correlates with how much fluid the pump can move in a given amount of time.

In a fixed displacement pump, the amount of fluid displaced with each rotation is constant. By increasing the speed of the prime mover, the pump rotates more frequently, which leads to a higher number of displacements over the same period. Consequently, this effectively increases the total volumetric output from the pump.

Understanding the mechanics of a fixed displacement pump underlines the significance of the prime mover's speed. The output rate can only increase if the operations of the pump are accelerated without changing the fixed volume it dispenses per rotation. Therefore, to achieve a greater flow rate, getting the prime mover to operate at a higher speed is essential.

The other choices reflect actions that do not contribute positively to achieving higher volumetric output in a fixed displacement system. Reducing the pump size could potentially decrease the output, increasing fluid viscosity could affect flow negatively, and decreasing the pump speed would inherently lead to lower volumetric output, contrary to the desired effect.