The rate of diffusion is inversely proportional to the square root of the relative molecular mass; in other words, lighter molecules diffuse faster than heavier ones. 

The relationship between diffusion rate and molecular mass is an inverse one. This means that as the molecular mass of a substance increases, its diffusion rate decreases. Conversely, substances with lower molecular masses will diffuse more quickly.

Relative molecular mass of :

(a) Propane, C\(_3\)H\(_8\) = (12 x 3) + (1 x 8) = 36 + 8 = 44

(b) Oxygen, O\(_2\) = 16 x 2 = 32

(c) Methane, CH\(_4\) = 12 + (1x4) = 12 + 4 = 16

(d) Ammonia, NH\(_3\) = 14 + (1 x 3) = 17

Since CH\(_4\) has the least molecular mass compared to other options, it has the highest diffusion rate.