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(B) $H$ückel's rule states that a planar,cyclic,fully conjugated system is aromatic if it contains $(4n + 2) \pi$ electrons,where $n = 0, 1, 2, \dots$

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$4$

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(B) $H$ückel's rule states that a planar,cyclic,fully conjugated system is aromatic if it contains $(4n + 2) \pi$ electrons,where $n = 0, 1, 2, \dots$Let's analyze each species:$(i)$ Naphthalene: $10 \pi$ electrons $(n=2)$,aromatic.$(ii)$ Cyclopentadienyl anion: $6 \pi$ electrons $(n=1)$,aromatic.$(iii)$ Cyclobutadiene: $4 \pi$ electrons ($n=1$ for anti-aromatic),not $H$ückel's rule.$(iv)$ Cyclopropenyl anion: $4 \pi$ electrons,anti-aromatic.$(v)$ Cyclopropenyl cation: $2 \pi$ electrons $(n=0)$,aromatic.$(vi)$ Cyclooctatetraene: $8 \pi$ electrons,non-planar,non-aromatic.$(vii)$ Anthracene: $14 \pi$ electrons $(n=3)$,aromatic.The species that obey $H$ückel's rule are $(i), (ii), (v),$ and $(vii)$.Thus,the total number is $4$.

Consider the following compounds/species:
The number of compounds/species which obey $H$ückel's rule is $..........$

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Consider the following compounds/species: The number of compounds/species which obey $H$ückel's rule is $..........$