The polydispersity index is

$\begin{array}{1 1}(a)\;\large\frac{\bar{M_w}}{M}&\\(b)\; \large\frac{\bar{M_w}}{\bar{M_n}}\\(c)\;\large\frac{(\bar{M_w})^2}{\bar{M_n}}\\(d)\; \large\frac{\bar{M_n}}{\bar{M_w}}\end{array}$

The ratio of mass average molecular mass to the number average molecular mass is called polydispersity index PDI.
PDI=$\large\frac{\bar{M_w}}{\bar {M_n}}$
This gives an idea about the homogeneity of a polymer.
Hence (b) is the correct answer.