The correct decreasing order of polarizability of halide ions is

The correct order of the polarizability of the anion is ${\mathrm{I}}^{-},{\mathrm{Br}}^{-},{\mathrm{Cl}}^{-},{\mathrm{F}}^{-}$.

Polarizability is defines as the ease with which the electron cloud of an atom or molecule is distorted. 
As the size of the anion increases, the charge to size ratio decreases and the polarizability increases.

Hence, the correct option is option (D) ${\mathrm{I}}^{-},{\mathrm{Br}}^{-},{\mathrm{Cl}}^{-},{\mathrm{F}}^{-}$.