Which type of propulsion system utilizes a gas under pressure?

The correct choice refers to a propulsion system that utilizes a gas under pressure, which is characteristic of a solid propellant system. In solid propellants, chemical energy is stored in a solid form and converted into thrust as the propellant combusts. The combustion generates hot gases that expand rapidly, creating high pressure within the combustion chamber. This high-pressure gas is then expelled through a nozzle, producing thrust according to Newton's third law of motion.

Solid propellant systems are commonly used in various applications, including military missiles and certain types of rockets, because they are generally simpler in design and can be stored for extended periods without the complications associated with handling liquid fuels.

The other options describe different concepts. Liquid propellant systems involve fuel and oxidizer in liquid form that are mixed and ignited, not specifically relying on gases under pressure. The term "combustible fluid" could refer to various fuel types, but it does not directly describe a propulsion system based on gas pressure like solid propellants do. Lastly, pneumatic systems use compressed air or gases for mechanical work but are not specifically designed for propulsion in the same way as solid propellants.