ANS:C - Mach number

Out of the options provided, the dimensionless quantity is Mach number. The Mach number is a dimensionless quantity that represents the ratio of the speed of an object (or fluid) to the speed of sound in the medium through which it is moving. It is defined as: Mach number=Speed of sound in the medium/Velocity of object​ Since both the velocity and speed of sound have units of length per time (such as meters per second), they cancel out in the ratio, resulting in a dimensionless value. Specific weight, viscosity, and density are physical properties that have units and dimensions associated with them. Specifically:

• Specific weight (or unit weight) has units of force per unit volume (such as newtons per cubic meter or pounds-force per cubic foot).
• Viscosity has units of force multiplied by time per unit area (such as pascal-seconds or poise).
• Density has units of mass per unit volume (such as kilograms per cubic meter or pounds per cubic foot).