The correct option is B 2:1
Rate of effusion for a gas (r)∝P√M, where P is pressure and M is molar mass of gas.
Since the rate of effusion for both gases is equal so,
P1√M1=P2√M2
⇒P1P2=√M1√M2
During effusion all conditions are identical so Pressure of gas (P)∝Moles of gas(n)∝Molecules of gas(N)
=N1N2⇒NSO2NCH4=√MSO2MCH4=√6416=21