Explanation

[Solution by Dr. Abubacker] The tetrahedral Co²⁺ complex has the smallest CFSE ($\Delta_t=\frac49\Delta_o$, and weak). High-spin octahedral Co²⁺ ($d^7$) has $-0.8\Delta_o$. Co³⁺ ($d^6$) forms low-spin complexes with a much larger $\Delta_o$ and CFSE ($-2.4\Delta_o$ plus pairing); en is a stronger-field ligand than NH₃, so [Co(en)₃]³⁺ is the highest. Answer (4).