c
\(R = \frac{{{u^2}}}{g} = 6km\)

\({R_{\max }} = \frac{{{u^2}}}{{g\,(1 + \sin \alpha )}}\)

\(\alpha = 30^o\)

\(\therefore {R_{\max }} = \frac{{{u^2}}}{{g\,(1 + \sin {{30}^o})}}\)

\(\therefore {R_{\max }} = \frac{2}{3}\left( {\frac{{{u^2}}}{g}} \right) = \frac{2}{3} \times 6 \)

\(\therefore {R_{\max }} = 4\,km.\)