The electron withdrawing group like −CH3, −C2H5 etc. increases the basicity when present in a compound. Thus, (a) is most acidic among the given compounds as it has two −C2H5 group, then comes (b) as it has one- CH3 group, then comes NH3. Aniline is least basic as its lone pair of electrons are involve in resonance. Thus, the basicity order is as follows A>B>C>D