The correct answer is A. The normal stress in a fluid will be constant in all directions at a point only if it is incompressible.
Incompressible fluids are those that cannot be compressed. This means that the volume of an incompressible fluid does not change, even when it is subjected to a force.
The normal stress in a fluid is the force per unit area that is perpendicular to the surface of the fluid. The normal stress in an incompressible fluid is constant in all directions at a point because the volume of the fluid does not change.
If the fluid is compressible, the volume of the fluid will change when it is subjected to a force. This will cause the normal stress to vary in different directions at a point.
The other options are incorrect because they do not affect the normal stress in a fluid.
Option B: The viscosity of a fluid is a measure of its resistance to flow. The viscosity of a fluid does not affect the normal stress in the fluid.
Option C: Friction is a force that opposes the relative motion of two surfaces. Friction does not affect the normal stress in a fluid.
Option D: A fluid at rest is a fluid that is not moving. The motion of a fluid does not affect the normal stress in the fluid.