What symbol represents the stalling speed at which the airplane is controllable?

The symbol that represents the stalling speed at which the airplane is controllable is VS. This is defined as the minimum speed at which the aircraft can maintain controlled flight without stalling during level flight, usually determined in a clean configuration (without flaps or landing gear extended). Understanding VS is crucial for pilots since it provides an important reference for safe operation, particularly during takeoff and landing phases where maintaining controllability is essential.

Other options refer to different conditions: VS1 denotes stall speed in a specific configuration (usually landing gear and flaps retracted), VSO indicates stall speed in a specified configuration with flaps being used, and VST is not a commonly recognized symbol in aviation for stall speed. Hence, while these terms relate to different stall speeds based on configurations and performance, VS specifically outlines controllability and is the correct reference for pilots concerning stall awareness in general flying scenarios.