What can be adjusted to vary the amount of fluid being pumped in a plunger pump?

The correct choice for varying the amount of fluid being pumped in a plunger pump is adjusting the pump speed. In a plunger pump, the flow rate is directly related to how quickly the plunger moves back and forth. By increasing the speed of the pump, you enable the plunger to cycle more frequently, thus imparting a greater volume of fluid being pumped over a given period. Conversely, decreasing the speed reduces the flow rate.

While adjustments to components such as the connecting rod, piston diameter, or intake valve may influence pump performance, they do not directly control the flow rate in the same way that pump speed does. The connecting rod helps convert rotary motion to reciprocating motion, but it’s the speed of this cycle that ultimately dictates fluid output. The piston diameter would change the volume moved with each stroke but not the frequency of those strokes. The intake valve's position can impact fluid flow at the entry point but does not modulate the overall pumping rate as effectively as the speed of the pump does. Thus, adjusting the pump speed is the most direct means of controlling the fluid output of a plunger pump.