The ring systems having following characteristics are aromatic.
(i) Planar ring containing conjugated π bonds.
(ii) Complete delocalisation of the π−electrons in ring system i.e. each atom in the ring has unhybridised p-orbital, and
(iii) Presence of (4n+2)
−electrons in the ring where n is an integer (n = 0, 1, 2,...........) [Huckel rule].
Using this information classify the following compounds as aromatic/nonaromatic.
Compound (A)
Above compound is Pyridine, the structure is planar and have conjugation of double bonds. The lone pair of nitrogen is in resonance with double bonds of benzene ring so there is complete delocalisation of electrons. As the number of 
-electrons is 6 it satisfies the Huckel’s rule for aromaticity. Pyridine is an aromatic compound.
Compound (B)
The carbon atom no. 3 is an sp3 hybrid, and according to valence bond theory sp3 hybridization follows tetrahedral geometry and hence the given compound is not planar even though it has 6 π electrons and delocalisation. Not a aromatic hydrocarbon.
Compound (C)
Cyclopent-2,4-dien-1-ide anion, has delocalised electrons alongwith the negative charge on the anion. The carbon atom with negative charge is always in sp3 hybridization which is not planar so thus the given compound is not aromatic.
Compound (D)
Cyclopent-2,4-dien-1-ylium cation, has a planar geometry as all carbon are sp2 hybridised. The charge is delocalised due to movement of 
-electrons moving the positive charge. But the number of 
electrons is 4, which does not satisfy the Huckel’s rules. Thus the compound (D) is non-aromatic.
Compound (E)
All atoms are sp2 hybridized so the structure is planar and has all 
electrons are delocalised over the entire ring. The number of 
electrons are 6 which fits in the HUckle’s rule for integer n=1, thus the given compound is aromatic.
Compound (F)
Cyclopropene cation is a planar structure with delocalised electrons. The number or 
electrons is 2, which satisfies Huckel’s rule for integer n=1 in the formula (4n+2) 
electrons
Compound (G) does not have a planar structure at one carbon and does not satisfy Huckel’s Rule, so it is non-aromatic.
Thus, structure (A, E, F) are aromatic and rest are non-aromatic.