Which is the most economical depth of flow in an open rectangular channel?

Correct option is C

​$$\begin{aligned}&\text{For a rectangular channel, the section is most efficient (economical) when:} \\&\boxed{y = \frac{b}{2}} \quad \text{or} \quad \boxed{y = 2R} \\[1em]&\text{This means:} \\&\bullet \ \text{The \textbf{depth of flow} = half the bottom width} \\&\bullet \ \text{Or equivalently, \textbf{depth} = 2 × \textbf{hydraulic radius}} \\[1em]&\text{So both of the following are true for the most economical rectangular channel:} \\&\bullet \ y = \frac{b}{2} \\&\bullet \ y = 2R\end{aligned}$$​​