For which of the following complexes, the energy of the T2 set is lower than the E set. So this question is actually easier than it appears. What they're really asking is to determine is which one of these complex ions represents an octahedral complex.
So remember to be octahedral you need your metal cation to be connected to six ligands. And if we look, the only option that has our metal cation connected to six ligands is option D.
Here we have cobalt 3 plus ion connected to six water molecules. This would give us an octahedral orientation, which in turn would mean that our T2 set of three orbitals is lower in energy than our E set of our pair of orbitals.