Which pair of gases has the same average rate of diffusion at 25° C?

Graham's law of diffusion states that the rate of diffusion is inversely related to the square root of the gas's molecular mass and directly proportional to its pressure. 

$\frac{{\text{ Rate }}_1}{{\text{ Rate }}_2}=\sqrt{\frac{{\mathrm{M}}_2}{{\mathrm{M}}_1}}$

${\mathrm{M}}_1$ is the molar mass of gas 1.
${\mathrm{M}}_2$ is the molar mass of gas 2.
Rate1 is the rate of diffusion of the first gas.
Rate 2 is the rate of diffusion for the second gas.

It states that the relationship between diffusion rate and molecule mass is inverse. Therefore, all that is left to do is check the molar mass of the provided pair to see if they are equal or not.

As the molecular mass of both carbon dioxide and nitrous oxide is 44 grams. So both will be having same rate of diffusion.