Comparing basic strength of nitrogen heterocycles
It is likely that the electron pair (from the HOMO) being donated to the acid is from a &pi; molecular orbital. Since the &pi; bond is rather weak, the &pi; electrons should be rather high in energy and is thus able to interact well with the LUMO of the acid. When nitrogen atoms are incorporated into the &pi; system, the energy of the HOMO is lowered due to the high electronegativity of N. Thus, incorporation of N atoms into the system would decrease the basicity of the molecular ion. As such, A and D would be inferior bases, compared to B and C. 

Choosing between A and D, and between B and C, we would have to base our criteria for the basicity on the extent of delocalisation. In D, the &pi; system is larger, thus greater delocalisation was achieved, with a lowering of the energy of the HOMO. Thus, D is likely to be less basic than A. Similarly, the extent of delocalisation is also larger in C, thus it would have a lower-energy HOMO, compared to B. Thus, B is likely more basic than C.

Therefore, option (2) is likely the correct order of basicity.