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$