Which of the following is not considered an aircraft?

The correct choice identifies a submarine as not being an aircraft. By definition, aircraft are vehicles designed for air travel that are capable of flight. This includes various types of vehicles that operate in the atmosphere, such as helicopters, balloons, and gliders.

Helicopters are rotorcraft that derive lift from rotating blades and can hover, make vertical takeoff and landing, and fly in various directions. Balloons are lighter-than-air vehicles that ascend and move with the wind, relying on buoyancy. Gliders are unpowered aircraft that soar through the skies utilizing thermal currents and other aerodynamics to stay aloft.

On the other hand, a submarine operates underwater and is specifically designed for marine exploration and military use. It is characterized by its ability to navigate beneath the surface of the water, which fundamentally separates it from aircraft, as submarines do not engage in flight within the atmosphere. Thus, it does not meet the criteria to be classified as an aircraft.