D is a non aromatic compound, which is different than an anti-aromatic compound. Anti-aromatic compounds (like c) are very unstable. This means that it would require too much energy to produce a carbocation and would be bad.
Take a look at the carbon on the bottom (opposite of the positive charge in your drawing). This carbon is bonded to 4 other atoms: 2 carbons and 2 hydrogens. This means that the atom has sp3 hybridized orbital. Aromatic and Anti-aromatic compounds need to be a complete system of p orbitals, but since this carbon atom does not have p orbitals it cannot be aromatic.
No......This molecule is fairly stable.....Non aromatic is fine.......ONLY when you get a specie that is Anti-Aromatic do we call it the KISS of DEATH !!! For example, if 2.chloro -1, 3-cyclopentadiene reacted in an SN1 reaction, we would get an Anti-Aromatic specie that would be horribly unstable. The molecule pictured above is decent since it is a secondary carbocation with resonance. However, it is not nearly as stable as a specie that is aromatic !
