What is Effective Atomic Number (EAN)

Question

$\begin{array}{1 1} \text{(a)EAN= [Z metal - (ox.state of the metal) + 2(coordination number of the metal)]} \\ \text{(b)EAN= [Z metal + (ox.state of the metal) + 2(coordination number of the metal)]} \\ \text{(c) EAN= [Z metal - (ox.state of the metal) + 4(coordination number of the metal)]} \\ \text{(d) EAN= [Z metal + (ox.state of the metal) + 3(coordination number of the metal)]}\end{array}$