The correct answer is option 'C', Mach Number.

The Ratio of inertia force to elastic force is known as the Mach Number. It is a dimensionless quantity that represents the relative speed of an object moving through a fluid medium, such as air or water, to the speed of sound in that medium.

Mach Number can be calculated using the formula:

Mach Number = Velocity/Speed of Sound

Inertia force refers to the force exerted by a moving object due to its mass and acceleration. Elastic force, on the other hand, refers to the force exerted by a deformed or stretched object that tries to restore it to its original shape. The ratio of these two forces gives us the Mach Number, which is an important parameter in fluid dynamics.

Explanation:

1. What is the Mach Number?
The Mach Number is a dimensionless quantity that represents the relative speed of an object moving through a fluid medium to the speed of sound in that medium. It was named after the Austrian physicist and philosopher Ernst Mach.

2. Inertia force and Elastic force:
- Inertia force: It is the force exerted by a moving object due to its mass and acceleration. Inertia force is proportional to the rate of change of momentum and is given by the equation F = ma, where F is the force, m is the mass, and a is the acceleration.
- Elastic force: It is the force exerted by a deformed or stretched object that tries to restore it to its original shape. Elastic force is proportional to the amount of deformation and is given by Hooke's Law: F = kx, where F is the force, k is the spring constant, and x is the displacement.

3. The Ratio of inertia force to elastic force:
The ratio of inertia force to elastic force gives us the Mach Number. This ratio represents the balance between the forces acting on an object moving through a fluid medium.

4. Significance of the Mach Number:
The Mach Number is an important parameter in fluid dynamics, especially in the study of compressible flow. It determines the type of flow around an object and is used to classify flows as subsonic, transonic, supersonic, or hypersonic.

5. Calculation of the Mach Number:
The Mach Number can be calculated by dividing the velocity of the object by the speed of sound in the fluid medium. It is given by the formula M = V/c, where M is the Mach Number, V is the velocity, and c is the speed of sound.

In conclusion, the ratio of inertia force to elastic force is known as the Mach Number. It represents the relative speed of an object moving through a fluid medium to the speed of sound in that medium. The Mach Number is an important parameter in fluid dynamics and is used to classify flows and study compressible flow.