What type of relay will disconnect the generator from the main bus when generator voltage drops below battery voltage?

The correct choice is a reverse-current relay. This relay plays a critical role in protecting the electrical system by ensuring that when the generator's voltage falls below the voltage of the battery, it will disconnect the generator from the main bus.

When the generator is unable to produce adequate voltage, it can create a situation where current could flow in the reverse direction—back into the generator from the battery. The reverse-current relay is designed to sense this condition and act to prevent potential damage to the generator and ensure operational integrity. By disconnecting the generator in this scenario, the relay protects both the generator and the electrical system, allowing the battery to provide the necessary power without interference.

The other options do not fulfill this specific function within the system. A short-circuit relay typically responds to excessive current caused by a fault, while an overload relay safeguards against excessive current and potential overheating issues in connected devices. An under-voltage relay would typically disconnect a load if the voltage falls below a certain threshold but does not specifically address the issue of generator voltage dropping below battery voltage in the context described in the question.