The correct option is A 1920 K
RMS speed of a molecule of mass M is given by
Vrms=√3RTM
RMS speed of oxygen is given by (VO2)=√3RTO2MO2 ......(1)
RMS speed of helium gas (VHe)=√3RTHeMHe ......(2)
Given that, RMS speed of oxygen gas and helium gas are same.
So, equating (1) and (2), we get
√3RTO2MO2=√3RTHeMHe
⇒TO2MO2=THeMHe
∴TO2=THe×MO2MHe=240×324=1920 K
Thus, option (a) is the correct answer.