c
\(\begin{array}{l}
Change\,in\,momentum\,along\,the\,wall\\
 = mv\cos {60^ \circ } - mv\cos {60^ \circ } = 0\\
Change\,in\,momentum\,perpendicular\,\\
to\,the\,wall\\
 = mv\sin {60^ \circ } - \left( { - mv\sin {{60}^ \circ }} \right)\\
 = 2mv\,\sin {60^ \circ }\\
\therefore Applied\,force
\end{array}\)

\(\begin{array}{l}
 = \frac{{Change\,in\,momentum}}{{Time}}\\
 = \frac{{2\,mv\sin {{60}^ \circ }}}{{0.20}}\\
 = \frac{{2 \times 3 \times 10 \times \sqrt 3 }}{{2 \times 0.20}} = 50 \times 3\sqrt 3 \\
 = 150\sqrt 3 \,newton
\end{array}\)