Maximum ${\mathrm{pK}}_{\mathrm{b}}$ value is of 

Maximum ${\mathrm{pK}}_{\mathrm{b}}$ (least basic) is (d) since, lone pair on
N-atom is used to delocalise $\mathrm{\pi }$ -electrons on two benzene
rings.

Least basic has maximum value of ${\mathrm{pK}}_{\mathrm{b}}$.

Since, in (d), lone pair of electrons over $\mathrm{N}$-atom are delocalised to two benzene rings, thus is least basic.