The ratio of inertial force to viscous force represents

• Let's consider about the motion of a metal ball that is lowered into a vertical column of fluid, say water. 
• The ball then encounters an opposing force known as the viscous force when it tries to move through the water. 
• Additionally, the water surrounding the ball is moving relative to the ball.
• The Reynolds number is therefore defined as the ratio of inertial forces to viscous forces.
• The Reynolds number is used to comprehend the turbulence or disruption in the fluid that results from an object trying to move through it and provides a better grasp of fluid dynamics.

Hence the correct answer is Reynolds number.