The correct answer is (a) 11 cm
Explanation: The perimeter of the circular cross section and the square cross section will remain the same. Perimeter = 44 cm. Side of square = 11 cm. Hydraulic diameter DH of the pipe is given by 4A/P, where A = Area of cross section and P = wetted perimeter. In case of a square DH = side. Hence, the hydraulic diameter is 11 cm.