In phenyl methanamine, the lone pair on nitrogen of -NH₂ group is localised and does not undergoes resonance.

(B), (C) and (D) are aromatic amines in which lone pair of electrons of N-atoms goes in resonance (+R effect) with the benzene ring. So, Lewis basicity or donation of lone of electrons of these amines will be decreased in comparison to (A).

+ I effects of two -CH₃ groups increases electron density on N atom while lone pair of N atom take part in resonance with the benzene ring and decreases electron density on N atom. Both this effect try to compensate each other.

+ I effects of one -CH₃ groups increases electron density on N atom while lone pair of N atom take part in resonance with the benzene ring and decreases electron density on N atom. Both this effect try to compensate each other.

It has no + I-effect on N-atom to increase electron density on the N atom which was decreased due to lone pair of N atom take part in resonance with the benzene ring. So, A is purely aliphatic 1°-amine. B is aromatic. 3°-amine with more aliphatic nature (for two -CH₃ groups). C is aromatic 2°-amine with less aliphatic nature (for one -CH₃ group).
D is purely aromatic 1°-amine.

Hence basicity order is A > B > C > D.