Polarisability of halide ions increases in the order:

Polarizability of halide ions increases in the order ${\mathrm{F}}^{-},{\mathrm{Cl}}^{-},{\mathrm{Br}}^{-},{\mathrm{I}}^{-}$
The size of the anion directly relates to its polarizability. The polarizability increases as the size of the anion increases and vice versa. Iodide ion has the largest size and hence the greatest polarizability.
Hence, the option D is correct.