Which is most polarisable by cation?

High polarizing power is attributed to the atoms and ions responsible for the significant distortion. High polarizing power refers to a tiny, strongly charged cation.
The polarizing power of a cation increases with decreasing radius.
The more massive the anion, the more polarizing power the cation will have over it when polarizing it.
${\mathrm{I}}^{-}$ is therefore the most polarizable by cation.
Hence, the option D is correct.