When do the three vent valves open to allow refueling?

The correct answer focuses on the operation of the refueling system in the Dassault Falcon 2000LXS. The three vent valves are designed to open when the lever in the refueling coupling bay is raised. This action allows air to escape from the fuel tank as it gets filled with fuel, preventing over-pressurization and allowing for a smooth refueling process. The system is integrated to activate the vent valves automatically with this lever movement, facilitating proper fuel intake and ensuring safety during refueling.

The other options do not accurately describe the operation of the vent valves. The valves do not open automatically with the fuel pumps on, as that would not provide the necessary control over air release during refueling. They also do not operate based on a specific fuel level, nor do they only activate through manual intervention separate from the designed lever operation in the coupling bay. Thus, the lever's function is crucial for the correct and safe refueling process.