According to the Valence Bond Theory (VBT) in coordination chemistry, the geometry of a complex is determined by the hybridisation of the metal atom's orbitals, which is dictated by the coordination number :

1. Coordination Number 4: Can result in two geometries. $sp^{3}$ hybridisation leads to a tetrahedral distribution ((iii)), while $dsp^{2}$ hybridisation results in a square planar distribution ((iv)) .
2. Coordination Number 5: $sp^{3}d$ hybridisation involves the mixing of one $s$, three $p$, and one $d$ orbital, resulting in a trigonal bipyramidal geometry ((i)) .
3. Coordination Number 6: Both $sp^{3}d^{2}$ (outer orbital) and $d^{2}sp^{3}$ (inner orbital) hybridisations result in an octahedral distribution of hybrid orbitals ((ii)) .

Matching these pairs: (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii).