The correct answer is: A. sound.
Mach number is a dimensionless quantity used to represent the ratio of the speed of an object to the speed of sound in a fluid. It is named after the Austrian physicist and engineer Ernst Mach, who first proposed its use in 1877.
The speed of sound is the distance that a sound wave travels in a given amount of time. It is typically measured in meters per second (m/s). The speed of sound depends on the medium through which it is traveling. For example, the speed of sound in air is about 343 m/s, while the speed of sound in water is about 1,500 m/s.
Mach number is often used to describe the speed of aircraft. For example, an aircraft flying at Mach 1 is flying at the speed of sound. An aircraft flying at Mach 2 is flying at twice the speed of sound.
Mach number is also used to describe the speed of spacecraft. For example, the Apollo 11 spacecraft reached a Mach number of 11.2 during its ascent to the Moon.
Mach number is not typically used to describe the speed of ships. This is because the speed of sound in water is much greater than the speed of ships. For example, the speed of sound in water is about 1,500 m/s, while the speed of a typical cruise ship is about 30 m/s.