ANS:D - elasticity.

The Mach number is the ratio of the inertia force to the elasticity. The Mach number (Ma) is a dimensionless quantity used to represent the speed of an object or flow relative to the speed of sound in the medium. It is defined as the ratio of the speed of the object or flow (V) to the speed of sound in the medium (a): Ma=V/a​ The inertia force of an object or flow represents its resistance to changes in motion, which is related to its mass and velocity. The elasticity, in this context, represents the speed of sound in the medium, which is related to the compressibility and elasticity of the medium. Therefore, the Mach number is the ratio of the inertia force to the elasticity of the medium. It indicates how the speed of the object or flow compares to the speed of sound in the medium.