ANS:A - viscosity

Reynolds number (Re) is the ratio of inertial forces to viscous forces in a fluid flow. It helps characterize the flow regime of a fluid, indicating whether the flow is laminar or turbulent. Therefore, Reynolds number is not the ratio of initial force and viscosity, elasticity, gravitational force, or surface tension. The correct understanding is that Reynolds number is the ratio of inertial forces to viscous forces, given by: Re= Inertial forces​ / Viscous forces In equations, it's often expressed as:    Re=μVL​/  Where:    

• ρ is the density of the fluid,
• V is the velocity of the fluid relative to the object,
• L is a characteristic linear dimension (like diameter),
• μ is the dynamic viscosity of the fluid.