Mach number is used in connection with speed of
(a). Sound
(b). Aircraft
(c). Spaceship
(d). Ships

- Hint: Aeronautical engineers use this term as a unit of speed. It is a quantity studied in fluid dynamics so it should be related to aerodynamics and aircrafts.

Complete step-by-step solution -
The Mach number (M) is a quantity that is defined as the ratio of the velocity of a body divided by the speed of sound in undisturbed medium through which the body is travelling. It is a dimensionless quantity and it mainly depicts the ratio of flow of speeds.
$M=\dfrac{u}{c}$
Here M denotes the Mach Number
 u is the velocity of aircraft
 c is the speed of sound in the medium

The correct solution is Aircraft.

Additional information:
The term Mach number is named after an Austrian Physicist and philosopher Ernst Mach. Since Mach is not a unit of measure rather it is a dimensionless quantity it is written as Mach followed by the number for example- Mach 2 and not 2 Mach. When we say an aircraft is moving at Mach 1 its speed at that moment is 343 m/s. If the Mach number is less than one it indicates subsonic flow and if it is greater than 1 it means supersonic flow.

Note: If we take an aircraft flying at say Mach 0.3 then is traveling at 30% of the speed of sound or if we say that a missile is travelling at Mach 2 is traveling at two times the speed of sound.