Which unit is responsible for regulating and monitoring the starter/generator?

The Generator Control Unit (GCU) plays a critical role in regulating and monitoring the starter/generator system in aircraft. Its primary function is to ensure that the generator operates within specified parameters, managing voltage output, frequency, and other operational characteristics essential for the electrical system's stability and reliability.

The GCU monitors the performance of the starter/generator, handling functions such as load sharing and protecting against issues like overvoltage or overcurrent conditions. This involves controlling the excitation of the generator, which is vital for maintaining the correct electrical output during various phases of engine operation, including starting and generating power once the engine is running.

In contrast, while other units like the Battery Control Unit and Power Management Unit are important for their specific roles in the overall electrical system, they do not directly manage the starter/generator's performance. The Starter Relay Unit primarily acts as a switch to engage or disengage the starter motor and is not responsible for the regulation and monitoring of the generator functions. Thus, the GCU's specialized function makes it the correct answer for this question.