Here it is
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Effective atomic number (EAN), it is the number of electrons surrounding the nucleus of central metal atom in a metal coordination complex.
It is sum of metal atom's electrons and the  electrons from the surrounding ligands for co-ordinate bonds. Each ligand will give 2 electrons and metal ion electrons are its atomic number -ON of metal.
In
1) It is Co2+ and 6 ligands = 25+12=37
2)) It is Mn and 5 ligands = 25+10=35
3) It is Fe2+ and 6 ligands = 24+12=36
3) It is Fe3+ and 6 ligands = 25+12=37

Now check which EAN value is closer to inert gas .