Hi guys,
I wonder if this has anything to do with Graham's law of gas effusion, which was:
(Rate of diffusion of gas A)(density of gas A)^1/2 = (rate of gas B)(density of gas B)^1/2
(the ^1/2 means square root).
At STP conditions, 1 mole of methane should have the same volume as 1 mole of helium, right? (this I'm not sure). If that's the case, then methane is 4x denser than helium (i.e. 4x heavier for the same volume). If that's the case, the Graham's law equation works out to methane diffusing half as fast as helium.
Maybe that's why the answer is D..

Anyway, being that it's been 7 years since I took general chem, somebody might want to check up on this because I might be wrong.
P.S. BTW Helium is a noble gas and does NOT normally exist in diatomic form-- Hydrogen does.