The correct answer is: D. none of the above.
The Prandtl mixing length is a length scale used in the turbulent flow equations to represent the average distance over which momentum is transported by eddies. It is not zero at the pipe wall, but is instead proportional to the distance from the wall. It is also not maximum at the pipe wall, but instead decreases with increasing distance from the wall. Finally, it is not independent of shear stress, but instead increases with increasing shear stress.
The Prandtl mixing length is a key parameter in the turbulent flow equations, and its value has a significant impact on the accuracy of the predictions made by these equations. The Prandtl mixing length is typically determined empirically, and there is no single value that is universally applicable. The value of the Prandtl mixing length will vary depending on the specific flow conditions, such as the Reynolds number and the type of boundary layer.
The Prandtl mixing length is a useful tool for understanding and predicting turbulent flow. However, it is important to keep in mind that it is an idealized model, and that the actual flow may deviate from the predictions made by the Prandtl mixing length equations.