Correct option is B
In a fluid at rest, there is no relative motion between adjacent fluid layers, which means that the velocity gradient is zero. Since shear stress in a fluid is directly proportional to the velocity gradient, the absence of such a gradient in a stationary fluid result in zero shear stress. This is a fundamental concept in fluid mechanics, as shear stress arises from the frictional forces between moving layers of fluid. Therefore, for a fluid at rest, shear stress is zero.
- Undefined: Shear stress is well-defined and measurable.
- Very large: Incorrect, as there is no motion to generate shear stress.
- Finite: Not applicable since shear stress is zero in a stationary fluid.
Thus, the correct answer is Zero.
