Which of the following compounds are aromatic?
(I)     (II)     (III)     (IV)  

Pyrrole has aromatic character because the lone pairs of electrons on the nitrogen atom is partially delocalized into the ring, creating a 4n + 2 aromatic system.

The cyclopentadienyl anion is a planar, cyclic molecule having a regular pentagonal structure. It consists of six π-electrons, thus obeying Huckel's Rule of aromaticity.