When Mach number is less than unity, the flow is called-

Correct option is D

Sol. The Mach number is defined as the ratio of the speed of the flow to the speed of sound in the same medium. When the Mach number is less than unity (M < 1), it indicates that the speed of the flow is less than the speed of sound, and such a flow is termed as sub-sonic. Sub-sonic flows are typically encountered in many practical applications, such as in the flight of commercial aircraft at cruising speeds, where the flow speed is lower than the speed of sound.