Which light signal would indicate that aircraft must stop immediately?

A steady red light signal is universally recognized as an instruction for aircraft to stop immediately. This signal is an important safety measure at airports, ensuring that aircraft do not proceed onto a runway or taxiway when it is not safe to do so. The steady nature of the red light signifies a clear and unambiguous command, allowing pilots to quickly recognize the need to halt their movements.

In contrast, other light signals, such as flashing white or steady green, convey different meanings that pertain to guidance and operational status, rather than an immediate stop. Flashing green typically indicates that the aircraft may proceed with caution, while a steady green signals that it is safe to take off or land. Therefore, the steady red light stands out as the critical signal for ensuring the safety of all operations on the airfield, making it essential for pilots to respond promptly to avoid any potential hazards or accidents.