(a) Structure (a) has two
π -bonds and one lone pair of electrons over
N -atom and has planer structure. Thus, is an aromatic compound.
Thus, is an aromatic compound.
(b) The structure (b) also contain two
π -bonds and one lone pair of electrons. Thus, follow Huckel's rule. The structure is cyclic and planer.
∴(b) is also an aromatic compound.
(c) The structure (c) has only
2pi -bonds and do not follow Huckel's rule. So, it not an aromatic compound.
(d) Structure (d) also contain only two
π -bonds, thus do not follow Huckel's rule and is not an aromatic compound.
∴(a) is the correct answer.