d
\(\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,Here,\,u = 0,g = 10\,m\,{s^{ - 2}},\,h = 20\,m\\
Let\,v\,be\,the\,veloctiy\,with\,which\,the\,stone\,hits\,the\,ground.\\
\therefore \,\,\,{v^2} = {u^2} + 2gh\\
or\,\,v = \,\sqrt {2gh} = \sqrt {2 \times 10 \times 20} = \,20\,m/s\,\,\,\,\left( {\,u = 0} \right)
\end{array}\)