If a pump is rated for a 15 psig pressure differential but is operating at 20 psig, what issue may be present?

The correct choice highlights a plausible issue where a pump is operating above its rated pressure differential. When a pump is rated for a specific pressure differential, such as 15 psig, and is operating at a higher level, like 20 psig, it indicates that the pump may be under undue stress or strain.

A clogged strainer can lead to increased pressure conditions because it obstructs the flow of fluid, which in turn forces the pump to work harder to overcome this resistance. This scenario can cause the pressure differential experienced by the pump to rise above the manufacturer's specifications. Operating in such conditions can potentially lead to pump failure or damage over time, making it a significant concern for a steamfitter or maintenance technician.

The other issues, although relevant in the context of pump operation, do not directly explain the cause of an increased pressure differential. For example, insufficient lubrication and worn motor bearings typically lead to operational inefficiencies or mechanical failure without directly causing an increase in pressure differential. Excessive vibration might indicate underlying issues but would not typically be the primary reason for being over the rated pressure differential. Thus, the clogged strainer aligns most closely with the problem described in the question.