c
\({v_{rms}} = \sqrt {\frac{{3RT}}{M}}  = {v_{rms}} \propto \frac{1}{{\sqrt M }}\)         [ \(\because T =\) constant]    

\( \Rightarrow \,\,\frac{{{{({v_{rms}})}_{{O_2}}}}}{{{{({v_{rms}})}_{{H_2}}}}} = \sqrt {\frac{{{M_{{H_2}}}}}{{M{o_2}}}} \, \Rightarrow \,\frac{{400}}{{{{({v_{rms}})}_{{H_2}}}}} = \sqrt {\frac{2}{{32}}}  = \frac{1}{4}\)

\( \Rightarrow \,\,\,{({v_{rms}})_{{H_2}}} = 1600\,\,m/s\)