Determine Total Number of Aromatic Compounds and Species

To determine the total number of aromatic compounds/species from the given structures, we apply Hückel's Rule of Aromaticity.

A compound or species is defined as aromatic if it satisfies the following criteria:

1. It is cyclic and planar.
2. It exhibits complete cyclic conjugation (every atom in the ring system possesses an unhybridized $p$-orbital, i.e., all ring vertices are $\text{sp}^2$ hybridized).
3. It contains $(4n + 2)$ $\pi$-electrons in the conjugated system, where $n = 0, 1, 2, 3, \dots$

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Step-by-Step Analysis of Each Structure:

1. Structure 1 ($p$-Benzoquinone / Cyclohexa-2,5-diene-1,4-dione):

   • It contains a 6-membered ring with two carbonyl group double bonds and two ring double bonds.
   • It lacks a continuous delocalized cyclic $\pi$-electron system within the ring.
   • Classification: Non-aromatic

2. Structure 2 (Tropylium Cation, $\text{C}_7\text{H}_7^+$):

   • Monocyclic, planar, 7-membered ring with 3 double bonds and one carbocation ($\text{sp}^2$ carbon with a vacant $p$-orbital).
   • All 7 carbons are $\text{sp}^2$ hybridized, providing complete cyclic conjugation.
   • Number of $\pi$-electrons = $6$
   • Fits Hückel's rule: $4n + 2 = 6 \implies n = 1$.
   • Classification: Aromatic

3. Structure 3 (Phenanthrene):

   • Fused polycyclic aromatic hydrocarbon composed of three benzene rings.
   • Entirely planar and completely conjugated across the perimeter.
   • Number of $\pi$-electrons = $14$
   • Fits Hückel's rule: $4n + 2 = 14 \implies n = 3$.
   • Classification: Aromatic

4. Structure 4 (Tetralin / 1,2,3,4-Tetrahydronaphthalene):

   • Consists of a benzene ring fused to a saturated 6-membered cyclohexane ring.
   • Due to the presence of $\text{sp}^3$ hybridized carbon atoms in the saturated ring, the complete bicyclic ring system is not fully conjugated throughout the entire molecule.
   • Classification: Non-aromatic (as a unified bicyclic conjugated system)

5. Structure 5 (Cyclobutene Dication, $\text{C}_4\text{H}_4^{2+}$):

   • A 4-membered ring with one $\text{C}=\text{C}$ double bond and two positive charges ($\text{sp}^2$ hybridized carbocations).
   • All 4 ring carbons are $\text{sp}^2$ hybridized, resulting in complete cyclic conjugation.
   • Number of $\pi$-electrons = $2$
   • Fits Hückel's rule: $4n + 2 = 2 \implies n = 0$.
   • Classification: Aromatic

6. Structure 6 (Cyclopentenyl Anion):

   • A 5-membered ring containing one double bond and a carbanion, but also contains saturated $\text{sp}^3$ carbon atoms.
   • The continuous cyclic conjugation is interrupted by the $\text{sp}^3$ carbons.
   • Classification: Non-aromatic

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Conclusion:

The aromatic species among the given structures are:

1. Tropylium cation
2. Phenanthrene
3. Cyclobutene dication

The total number of aromatic compounds/species is 3.

Correct Option: C