Chlorobenzene is formed by the reaction between benzene and...

The reaction of benzene with chlorine in the presence of $\text{AlCl}_3$ is an electrophilic aromatic substitution.

The Lewis acid $\text{AlCl}_3$ reacts with $\text{Cl}_2$ to generate the chloronium ion, $\text{Cl}^+$, which acts as an electrophile.

$\text{Cl}_2 + \text{AlCl}_3 \rightarrow \text{Cl}^+ + \text{AlCl}_4^-$

The electrophile $\text{Cl}^+$ then attacks the electron-rich benzene ring to form a resonance-stabilized carbocation (sigma complex).

Thus, the species that actually attacks the benzene ring is $\text{Cl}^+$.

Answer: $\text{Cl}^+$