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According to Lewis theory, an electron pair donor is called as a base.

In the compounds C and D the nitrogen atom is attached to theelectronegativecarbon atom. The lone pair present on the nitrogen atom is delocalizing on the benzene ring. So, availability of lone pair electron for protonation is very less.

The nitrogen atom in compound-D is attached to the less electronegativecarbon atom.

The nitrogen atom in compound-A ishybridized. Whereas, in compoundD it is attached to the less electronegativecarbon atom.

Thus, the strongest base is compound-D (benzylamine).

Sp² nitrogen is more electronegative than an Sp³ nitrogen and the more electronegative the atom to which a hydrogen is attached, the stronger the acid. the stronger the acid, the weaker is its conjugate base.