Solving this problem uses an equation that contains molecular mass and the speed, or perhaps diffusion rate.
If you can write the equation from your book/notes here, we could look at it together (rather than me trying to guess in what form your book/notes presents the information )
If you use the relative molecular mass formula, you will find that the relative molecular mass of methane is 16, oxygen is 32, nitrogen is 28 and C2H6 is 30. From these, i think you would know which gas diffuses the fastest and the slowest. Also, if you look at the atomic mass of the elements used to make up the different molecules/compounds, you will also find out how to get the relative molecular mass.
*note: i am quite new to this subject so please feel free to correct me if i am wrong. thank you.