Which solution has the greatest percent ionization?

Ionization constant α to find :

$\alpha =\sqrt{\frac{k_a}{c}}$; Highest is for b.

${\mathrm{K}}_{\mathrm{b}}=1.8\times {10}^{-5}\mathrm{C}=0.1\mathrm{M}$

${\alpha }^2=\frac{{\mathrm{K}}_{\mathrm{b}}}{\mathrm{c}}=\frac{1.8\times {10}^{-5}}{0.1}=1.8\times {10}^{-4}$

$\Rightarrow \alpha =0.0134$

∴    Percentage ionization =1.34% which is highest of the four values given.