a
\(\,{\upsilon _{rms}}\sqrt {\frac{{3RT}}{M}}  \Rightarrow {\upsilon _{rms}} \propto \sqrt {\frac{T}{M}} \,\,\,\,\,\,\,\,\,\,\,\,\,[\because {\upsilon _{rms}}R \to {\text{constant]}}\,\,\,\)

\( \Rightarrow \frac{{{{\text{T}}_{{{\text{H}}_{\text{2}}}}}}}{{{{\text{T}}_{{{\text{O}}_2}}}}} = \frac{{{M_{{H_2}}}}}{{{M_{{O_2}}}}} \Rightarrow \frac{{{T_{{H_2}}}}}{{(273 + 47)}} = \frac{2}{{32}} \Rightarrow {T_{{H_2}}} = 20\,K\)