Explanation

(b)
Basicity of amines increases with increasing +I effect of alkyl group.3${^\circ}$amine has greater +I effect than 2${^\circ}$ and 1${^\circ}$amines but less basic than these, due to steric hindrance of bulky groups. Moreover, benzyl amine is a weaker base than aliphatic amines. Hence, the following compounds has the order of basicity.
$CH_{3}NH_{2}(1{^\circ}),\left( CH_{3} \right)_{2}NH(2{^\circ}),\ \left( CH_{3} \right)_{3}N(3{^\circ}),\ C_{6}H_{5}CH_{2}NH_{2}$
I II III IV
II > I > III > V