With what device is the RPM of an AC generator indirectly measured on a switchboard when operating singly (no other generator connected to the bus)?

Correct Answer: Option A — frequency meter

When an AC generator operates singularly on an isolated bus, its terminal frequency is directly proportional to its rotational speed. Under IEEE 45, the switchboard frequency meter serves as the primary visual indicator of alternator RPM, as frequency fluctuates precisely with the prime mover's speed.