According to Graham's law , at a given temperature , the ratio of rates of diffusion $\large\frac{r_A}{r_B}$ of gases A and B is given by:

$\begin {array} {1 1} (a)\;(\large\frac{P_A}{P_B})(\large\frac{M_A}{M_B})^{\frac{1}{2}}\\(b)\;(\large\frac{M_A}{M_B})(\large\frac{P_A}{P_B})^{\frac{1}{2}}\\(c)\;(\large\frac{P_A}{P_B})(\large\frac{M_B}{M_A})^{\frac{1}{2}}\\(d)\;(\large\frac{M_A}{M_B})(\large\frac{P_B}{P_A})^{\frac{1}{2}} \end {array}$

According to Graham's law
We have
$\large\frac{r_1}{r_2} = \sqrt{\large\frac{M_2}{M_1}}\times\large\frac{P_1}{P_2}$
Hence we can write as
$=(\large\frac{P_A}{P_B})(\large\frac{M_B}{M_A})^{\frac{1}{2}}\qquad$