In the presence of six ligand units the degeneracy of d-orbitals is lost and split into two sets. The pair of orbitals present in a single set are

The six lingad units are divided into two groups of 3 and 2. 

In the 3 unit group includes ${\mathrm{d}}_{\mathrm{xy}},{\mathrm{d}}_{\mathrm{yz}}\mathrm{and} {\mathrm{d}}_{\mathrm{zx}}$

In the 2 unit group includes ${\mathrm{d}}_{z^2}\text{ and }{\mathrm{d}}_{{\mathrm{x}}^2-{\mathrm{y}}^2}$

Therefore the pair of orbitals present in a single set is option C.