What action does the runway controller take to allow take off?

The runway controller displays a steady green lamp to authorize an aircraft for takeoff. This visual signal is part of the standardized procedures in air traffic control, indicating to pilots that it is safe to begin their departure from the runway. The steady green light is clearly visible and understood universally among pilots, ensuring a smooth communication process that is vital for flight safety.

Other potential options involve actions that do not indicate permission for takeoff. For example, a continuous alarm is typically used to signal a warning or alert, not to provide clearance. A flashing yellow light generally indicates caution, prompting pilots to be prepared for potential hold or re-evaluation of conditions. An orange flag is not a recognized signal in this context, as it does not convey the necessary imperative for takeoff. Therefore, displaying a steady green lamp is the correct action that allows for a safe and clear takeoff.