The correct answer is: C. when there is no motion of one fluid layer relative to an adjacent layer.
The normal stress is the component of the stress tensor that is perpendicular to the surface on which it acts. It is a measure of the force per unit area that is exerted on the surface by the fluid.
The normal stress is the same in all directions at a point in a fluid when there is no motion of one fluid layer relative to an adjacent layer. This is because the fluid is in equilibrium and there are no forces acting on it to cause it to move.
When the fluid is moving, the normal stress is not the same in all directions. This is because the fluid is not in equilibrium and there are forces acting on it to cause it to move. The direction of the force depends on the direction of the motion of the fluid.
The normal stress is also not the same in all directions when the fluid is viscous. This is because the viscosity of the fluid causes the fluid to resist changes in motion. The direction of the force depends on the direction of the change in motion of the fluid.
A. only when the fluid is frictionless is incorrect because the normal stress is the same in all directions at a point in a fluid when there is no motion of one fluid layer relative to an adjacent layer, even if the fluid is not frictionless.
B. only when the fluid is incompressible and has zero viscosity is incorrect because the normal stress is the same in all directions at a point in a fluid when there is no motion of one fluid layer relative to an adjacent layer, even if the fluid is not incompressible and has non-zero viscosity.
D. irrespective of the motion of one fluid layer relative to an adjacent layer is incorrect because the normal stress is not the same in all directions when the fluid is moving.