Which of them have highest covalent character?

Fajan's rule establishes the covalent nature of a molecule.
According to Fajan's rule, the covalent nature of the molecule rises as the charge density on the cation increases.
The cations of ${\mathrm{SiCl}}_4,{\mathrm{MgCl}}_2,\mathrm{NaCl},\mathrm{KCl}$ are ${\mathrm{Si}}^{+4},{\mathrm{Mg}}^{+2},{\mathrm{Na}}^{+},{\mathrm{K}}^{+}$ respectively.
So, ${\mathrm{SiCl}}_4$  has the maximum covalent character.
Hence, the option D is correct.