Consider the four cyclic species labeled (I) to (IV) in the given figure. According to Huckel's rule, which of the following combinations correctly identifies ONLY the aromatic species?

Step-by-step solution

According to Huckel's rule, a species is aromatic if it is cyclic, planar, completely conjugated, and has (4n + 2) electrons, where n = 0, 1, 2, For species (I), pyrrole: It is a cyclic, planar molecule. The nitrogen atom is sp^2 hybridized and its lone pair resides in a p-orbital, participating in conjugation. Total electrons = 4 (from two double bonds) + 2 (from lone pair) = 6 electrons. Since 6 = 4(1) + 2 , it follows Huckel's rule and is aromatic. For species (II), cyclopropenyl anion: It is cyclic and planar.