The PTU is a reversible motor pump located between the:

The PTU, or Power Transfer Unit, is specifically designed to operate between the Green and Yellow hydraulic systems. Its primary function is to assist in maintaining hydraulic pressure and ensuring that both hydraulic systems can work together harmoniously, especially in cases where one system is underperforming or has a failure.

The PTU allows for the transfer of hydraulic fluid from the Yellow system to the Green system, enabling the aircraft to maintain critical flight control and landing gear functions even if one of the hydraulic systems has lost pressure. It acts as a backup to ensure the operation of essential systems, which is vital for safety during flight.

In contrast, the other options refer to combinations of hydraulic systems that do not involve the PTU's role. The PTU operates specifically to link the Green and Yellow systems, making that option the correct answer.