ANS:A - less than 2100

Flow in pipes is considered laminar if the Reynolds number is less than 2100. The Reynolds number (Re) is a dimensionless quantity used to predict the flow regime in fluid mechanics. It's defined as the ratio of inertial forces to viscous forces within a fluid flow. The formula for the Reynolds number is: Re=ρVD​/μ  Where:

• ρ is the fluid density,
• V is the fluid velocity,
• D is a characteristic length (usually the diameter of the pipe),
• μ is the dynamic viscosity of the fluid.

For flow in pipes, if the Reynolds number is less than 2100, the flow is generally laminar, meaning the fluid flows smoothly in layers without significant mixing. If the Reynolds number exceeds 2100, the flow is considered turbulent, characterized by chaotic and unpredictable motion of fluid particles. Therefore, the correct option is less than 2100.