If the temperature of a gas is increased from 27∘ C to 927∘C, the root mean square speed of its molecules
is doubled
The root mean square speed is given by
Vrms=√3kTm
i.e. Vrms∝T. Initial temperature T1=27+273=300K. Final temperature T2=927+273=1200K. Since temperature is increased by 4 times, the speed is doubled. Hence the correct choice is (b)