What action is required to manually transfer fuel from the center tank to the wing tank?

Engaging the XFLOW system is the correct action required to manually transfer fuel from the center tank to the wing tank. The XFLOW system, or crossflow system, is specifically designed to facilitate the movement of fuel between the center tank and the wing tanks, allowing for better management of the aircraft's fuel distribution.

When operating the XFLOW system, fuel can be transferred without the need for additional pumps, relying on gravity and the designed flow paths in the fuel system. This process ensures that fuel is used efficiently, balancing the load between the tanks to optimize aircraft performance and safety.

The other choices involve different functionalities within the fuel system. Activating the left fuel pump primarily serves to supply fuel from the left tank and does not directly initiate the transfer from the center tank. Opening fuel valves could refer to enabling fuel flow but does not specify the mechanism for transferring fuel as the XFLOW system does. Crossfeeding from the right tank deals with redistributing fuel from one tank to another to supply the engine, rather than transferring fuel between the center and wing tanks directly. Thus, engaging the XFLOW system is the correct and specific method for transferring fuel as stated in the question.