${\mathrm{K}}_{\mathrm{b}}$ is least for

$\mathrm{K}{}_{\mathrm{b}}\propto \mathrm{Basicity} \mathrm{of} \mathrm{compound}$

And, Basicity of amines $\alpha$ Inductive effect

Thus, order of basicity will be,

$\mathrm{Ph}-{\mathrm{NH}}_2<{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2{\mathrm{NH}}_2<{\mathrm{C}}_2{\mathrm{H}}_5-{\mathrm{NH}}_2<{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2-\mathrm{NH}$

Therefore, $\mathrm{Ph}-{\mathrm{NH}}_2$ is the least basic and as a result it has least $K{ }_b$

Hence, option (A) is correct.