Out of the following which one has the highest values of covalent character?

${\mathrm{HgCl}}_2$ has the highest values of covalent character.
The ionic size increases in the order ${\mathrm{Zn}}^{2+}<{\mathrm{Cd}}^{2+}<{\mathrm{Hg}}^{2+}$
The covalent nature grows with size as the effective nuclear charge increases. While mercuric chloride is covalent in nature, zinc and cadmium chlorides are ionic in form.
Hence, the option C is correct.