a
\(\begin{array}{l}
R = \frac{{{u^2}{{\sin }^2}\theta }}{g},H = \frac{{{u^2}{{\sin }^2}\theta }}{{2g}};{H_{\max }}at2\theta  = 90\\
{H_{\max }} = \frac{{{u^2}}}{{2g}}\,; = 10 \Rightarrow {u^2} = 10g \times 2\\
R = \frac{{{u^2}\sin 2\theta }}{{\left( g \right)}} \Rightarrow {R_{\max }} = \frac{{{u^2}}}{g}\\
{R_{\max }} = \frac{{10 \times g \times 2}}{g} = 20\,meter
\end{array}\)