The molecular formula of Mercury (II) chloride is: 

Ionic compounds are formed between a metal and a non-metal. While naming an ionic compound, the name of the cation is written first as its base name with its oxidation state in the brackets, and then comes the name of the anion (ending with ‘ide’).

So, Mercury (II) chloride is an ionic compound in which mercury (II) ions $\left({\mathrm{Hg}}^{2+}\right)$are the cations and Chloride $( {\mathrm{Cl}}^{-})$ion is the anion. As per the crisscross method, while writing the ionic formula of any compound, the numerical value of each of the ionic charges crosses over to become the subscript of the other ion.

Thus, using the method, the subscript of Hg becomes 1, and the subscript of Cl becomes 2.

${\mathrm{Hg}}^{+2}+ 2{\mathrm{Cl}}^{-1}\to {\mathrm{HgCl}}_2$

Hence the correct option is (D) ${\mathrm{HgCl}}_2$