What type of propulsion might an airplane use?

The correct choice is C, as propeller or jet engines are the primary means of propulsion used in modern airplanes. Propellers convert rotary motion from an engine into thrust, allowing planes to fly by pushing against the air. Jet engines, on the other hand, operate based on the principles of jet propulsion, expelling high-speed exhaust gases to generate thrust in accordance with Newton's third law of motion. This combination of propulsion methods is foundational to modern aviation, enabling various aircraft to achieve the necessary lift and speed for flight.

Other options, while interesting, do not represent standard propulsion systems used for airplanes. Human power is impractical for the majority of aircraft due to the required speed and energy, and wind power is not a reliable or effective means of propulsion for powered flight, although some gliders can utilize wind currents. Water current systems are not applicable to aviation, as they pertain to marine craft rather than aerial vehicles.