ANS:A - less than 500

In open channel flow, the flow is considered laminar if the Reynolds number is less than 500. The Reynolds number (Re) is a dimensionless quantity used to predict the flow regime in a fluid flow situation. It is defined as: Re=ρ⋅V⋅L​/μ  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 (such as diameter for a pipe or depth for an open channel), and
• μ is the dynamic viscosity of the fluid.

In open channel flow, if the Reynolds number is less than 500, the flow is typically laminar. In laminar flow, the fluid flows smoothly in parallel layers with little mixing between layers. If the Reynolds number exceeds 500, the flow may transition to turbulent flow, where the flow becomes chaotic and irregular. Therefore, the correct statement is: less than 500.