Which of the following is a requirement for onboard generators intended to be paralleled?

Generators that are intended to be paralleled must be synchronized before being connected to the same electrical bus. Synchronization ensures that the voltage, frequency, and phase of the generators match precisely. If generators are not synchronized when paralleled, it can lead to severe electrical disturbances, including damaging transients and system failure. This is critical for the proper functioning of the system, as it allows multiple generators to operate together effectively, sharing the load without causing issues such as circulating currents or power disputes between them. Each generator operates harmoniously, maintaining system stability and reliability.

The other options do not reflect essential requirements for paralleling generators. While having a backup generator is good for redundancy, it's not a requisite for paralleling. Similarly, generators do not need to be from the same brand or of equal size to parallel. Different brands can have compatible output specifications, and varying sizes can be paralleled as long as they are properly synchronized and capable of handling the load requirements.