Assertion (A): Equal moles of helium and oxygen gases are given equal quantities of heat at constant volume. There will be greater rise in the temperature of helium compared to that of oxygen.

Reason (R): The molecular weight of oxygen is greater than the molecular weight of helium.

Relation between heat and change in temperature can be given by $\mathrm{Q} = {\mathrm{nC}}_{\mathrm{v}}∆\mathrm{T}$

Therefore for equal quantity of heat, rise in temperature is indirectly proportional to C_v.

${\mathrm{C}}_{\mathrm{v}} \propto \frac{1}{\mathrm{T}}$

Oxygen is diatomic where as helium is monoatomic. Hence C_v of oxygen is higher than that of helium. 

So, rise in temperature is higher for helium. 

Molecular weight of oxygen is greater than the molecular weight of helium but that is not the reason for the higher rise of temperature in the case of helium.