The degree of dissociation $'\alpha'$ of a weak electrolyte is (N is number of ions given by 1 mol of the electrolyte)

Question

$\begin{array}{1 1}(a)\;\large\frac{i-1}{N-1}\\(b)\;\large\frac{i-1}{N+1}\\(c)\;\large\frac{N+1}{i-1}\\(d)\;\large\frac{N-1}{i-1}\end{array}$