At the centre line of a pipe flowing under pressure where the velocity gradient is zero, the shear stress will be A. minimum B. maximum C. negative value D. could be any value

The correct answer is A. minimum.

The shear stress is the force per unit area acting parallel to the surface of a fluid. It is caused by the relative motion of the fluid particles. The shear stress is zero at the center line of a pipe flowing under pressure because there is no relative motion of the fluid particles at the center line.

Option B is incorrect because the shear stress is minimum at the center line of a pipe flowing under pressure.

Option C is incorrect because the shear stress cannot be a negative value.

Option D is incorrect because the shear stress is not any value. It is a specific value that is determined by the fluid properties and the flow conditions.