How are external tanks equipped to manage fuel flow?

External fuel tanks are equipped with control valves and pumps to effectively manage the transfer of fuel to the aircraft's fuel system. This system ensures that fuel can be moved from the tanks to the engines in a controlled manner, taking into account factors such as fuel pressure and demand from the engines. Control valves regulate the flow of fuel, while pumps help to create the necessary suction or pressure to draw fuel into the fuel lines.

Using this combination allows for precise control over the fuel transfer process, which is essential for maintaining engine performance and ensuring safety during flight operations. Without these mechanisms, managing fuel flow would be less efficient and could lead to fuel starvation or imbalance.

The other options do not adequately describe the mechanisms used for managing fuel flow in external tanks. Gravity could be part of the process in some simple systems but is not a reliable means for all aircraft. Engine pressure is not a method used for transferring fuel from external tanks. Built-in fuel gauges help monitor fuel levels but do not control fuel flow.