Which of the pairs of gases diffuse at a slower rate than ${\mathrm{CO}}_2$?

We know that the rate of diffusion $\propto \frac{1}{\sqrt{\text{ Molecular weight }}}$

So, the gases with their molecular weight greater than that of ${\mathrm{CO}}_2(\mathrm{MW}=44)$

would diffuse at a slower rate. 

Hence,${\mathrm{SO}}_2$ and ${\mathrm{SO}}_3$ will be diffusing at a rate slower than of ${\mathrm{CO}}_2$.