Which of the following does not exist?

${\mathrm{SCl}}_6$ does not exist.

The d orbitals of S must participate in hybridization for ${\mathrm{SCl}}_6$ to develop $\left(s{p}^3{d}^2\right)$. The energy of sulfur's d orbitals is sufficiently decreased in the presence of electronegative elements like F for the electrons to be promoted, but the electronegativity of Cl relative to sulphur is not high enough to drop the d orbital's energy to the level needed to stabilize the system. Additionally, there will be too much steric strain if sulphur is surrounded by six large Cl atoms.

Hence, option D is correct.