Correct Answer is: (b) 36
Effective atomic number = normal atomic number - oxidation state of the atom + (2 x coordination number)
So here, the atom is Cobalt (Co)
Atomic number =27
Oxidation state of Co in this compound is +3
(Because of 3 Cl atoms)
Coordination number is 6
So EAN = 27 - 3 + (2 x 6)
= 24 + 12
= 36