a
\(\begin{array}{l}
Time\,taken\,by\,a\,body\,fall\,form\,a\,hight\,h\,\\
to\,reach\,the\,ground\,is\,t = \sqrt {\frac{{2h}}{g}.} \\
\therefore \,\frac{{{t_A}}}{{{t_s}}} = \frac{{\sqrt {\frac{{2{h_A}}}{g}} }}{{\sqrt {\frac{{2{h_g}}}{g}} }} = \sqrt {\frac{{{h_A}}}{{{h_B}}}} = \sqrt {\frac{{16}}{{25}}} = \frac{4}{5}.
\end{array}\)