d
$E \propto T$
$\therefore \frac{{{E_2}}}{{{E_1}}} = \frac{{{T_2}}}{{{T_1}}} = \frac{{600}}{{300}} = 2$
$\therefore \,\,{E_2} = 2{E_1}\,\,\,$
$\therefore \,\,\,{E_2} = 2 \times 6.21 \times {10^{ - 21}}\,\, = 12.42 \times {10^{ - 21}}\,J$
${\upsilon _{rms}} \propto \sqrt T $ પરથી $\frac{{{{({\upsilon _{rms}})}_2}}}{{{{({\upsilon _{rms}})}_1}}} = \sqrt {\frac{{{T_2}}}{{{T_1}}}} = \,\sqrt {\frac{{600}}{{300}}} \, = \,\sqrt 2 \,\,$
$\therefore {({\upsilon _{rms}})_2}\, = \,\sqrt 2 {({\upsilon _{rms}})_1}$
$\therefore {({\upsilon _{rms}})_2}\,\, = \,\sqrt 2 \times 325\,\, = \,459.6\,m/s$