A complex involving ${\mathrm{d}}^2{\mathrm{sp}}^3$ hybridization is

A complex involving ${\mathrm{d}}^2{\mathrm{sp}}^3$ hybridization is an octahedral geometry.

When six atoms, groups of atoms, or ligands are symmetrically positioned around a central atom to form the vertices of an octahedron, the compound is said to have an octahedral molecular geometry, also known as square bipyramidal. The prefix octa refers to the eight faces of the octahedron.

Hence, option C is correct.