ANS:A - d/4

The hydraulic radius Rh​ of a circular pipe flowing full is defined as the cross-sectional area divided by the wetted perimeter. For a circular pipe of diameter d, the cross-sectional area (A) is given by the formula for the area of a circle: A=πd2/4​ The wetted perimeter (P) is simply the circumference of the circular pipe: P=πd Therefore, the hydraulic radius (Rh​) is given by: 4Rh​=A/P​=πd4πd2​​=4d​ So, the hydraulic radius of a circular pipe flowing full is d/4​.