Answer:
D. Flow separation is caused due to adverse pressure gradient in the flowing fluid.
Explanation:
Flow separation :
When adverse pressure gradient become dominate then phenomenon of flow separation occurs.In the other words when boundary layer is form against the adverse pressure then phenomenon of flow separation occurs.The adverse pressure means a opposing which act in the opposite to the direction of fluid flow.Due to flow separation eddy formation occurs and these eddy leads to increases the losses in the fluid flow.Due to flow separation fluid leaves the solid surface and form eddies.
So the answer is D.
