Correct option is B
The velocity of a fluid within the boundary layer starts from zero at the surface and gradually increases to the free-stream velocity because of the no-slip condition imposed by the surface. This condition states that fluid in immediate contact with a solid boundary has zero velocity relative to the surface. Moving away from the surface, viscous effects gradually allow the velocity to increase until it matches the free-stream flow. Turbulence, pressure gradients, and acceleration influence the boundary layer development but do not explain the zero velocity at the wall.
