What does a steady red light signal mean when directed to an aircraft in flight?

Answer: B — Stop

A steady red light signal directed to an aircraft in flight indicates that the pilot must stop. This signal serves as a critical alert for aircraft to cease their current operation, often due to safety reasons, such as obstacles on the runway or in the surrounding airspace. The clarity and immediacy of the red signal emphasize the urgency for the pilot to halt all actions until further communication or instructions are given from air traffic control. The other options, typically associated with clearances or maneuvers, do not apply in this context, as the steady red light specifically denotes the need to stop.