The ratio of the hydraulic radius of a pipe running full to the hydraulic radius of a square section of a channel whose side is equal to the diameter of the pipe, is

ANS:D -

The hydraulic radius of circle = d/4.
The hydraulic radius of square = d/3 ( because here we are considering wetted perimeter so two sides and bottom are wetted but top width is not wetted because it's open in the atmosphere)
So, correct answer is = d.3/d.4 = 3/4.