Changing an impeller's diameter affects which of the following?

Changing the diameter of an impeller has a significant impact on both pump capacity and pressure. When the diameter of the impeller is increased, the capacity of the pump typically increases as well, allowing it to move a greater volume of fluid. This is because a larger diameter generates more energy, which can translate to higher flow rates.

On the other hand, the pressure generated by the pump can also change as a result of the altered diameter. An increased impeller diameter can lead to a rise in the pump's head, thereby increasing the pressure output. Conversely, decreasing the diameter can reduce both capacity and pressure. Therefore, the modification of the impeller diameter directly influences both pump capacity and pressure, which confirms the accuracy of this choice.