What does this ratio indicate :-$\bar{M_w}/ \bar{M_n}$

$\begin{array}{1 1}(a)\;\text{Polymer index}&(b)\; \text{Polyspersity index}\\(c)\;\text{Polydispersity index}&(d)\;\text{Polymolecular ratio}\end{array}$

The ratio of mass average molecular mass to the number average molecular mass is called polydispersity index(PDI).
PDI =$\bar{M_w}/ \bar{M_n}$
Hence (c) is the correct answer.