Given the same conditions of temperature and pressure, which noble gas will diffuse most rapidly a. he b. ne c. ar d. kr

Given the same conditions of temperature and pressure the noble gas He will diffuse most rapidly. He is the smallest noble gas and so it can diffuse (pass through) membranes the easiest.It is colorless, odorless, tasteless, non-toxic chemical element. 
Correct A

At the same conditions of temperature and pressure the noble gas with the smallest density will diffuse faster. But since density is proportional to the molecular mass. A look at the periodic table shows that Helium will diffuse faster. The correct answer is A.

Your question is not clear but if you can explain yourself i would be glad to .  just trying to : ))