Comparing SO2 (64.0g/mol) and CH4 (16.0g/mol) at the same temperature,
CH4 diffuses:
a. 1/4 as fast as SO2
b. 2 times as fast as SO2
c. 1/2 as fast as SO2
d. four times as fast as SO2

We know that Rate2/Rate1 = (M1/M2)^1/2.
Chemical Principles, fourth edition, Dickerson, Gray, Darensbourg, and
Darensbourg, The Benjamin/Cummings Publishing Company, Inc., 1984,
page 110.

Therefore, Rate CH4 = (64/16)^1/2*RateSO2 = 2*RateSO2

The answer is B.

Sincerely,

Wonko