Mach numbers are used primarily in relation to the velocity of objects moving through a fluid, such as air, and are most commonly applied to aircraft. The Mach number represents the ratio of an object’s speed to the speed of sound in the same medium. For example, an aircraft traveling at Mach 1 moves at the speed of sound, while at Mach 2, it travels twice as fast as sound. This concept is crucial in aerodynamics, influencing the design, performance, and behavior of aircraft, particularly at high speeds where compressibility effects and shock waves become significant. While Mach numbers can also apply to other high-speed vehicles like spacecraft, their primary and most frequent use is in the context of aircraft. Therefore, the correct answer is [C] Aircraft.