C₆H₅CHO is formed when C₆H₆ is treated with CO and HCl in the presence of anhydrous AlCl₃. Name of the reaction is:

The correct answer is Gattermann Koch reaction.

Explanation:
When benzene ($C_6{H}_6$) is treated with carbon monoxide ($\mathrm{CO}$) and hydrogen chloride ($\mathrm{HCl}$) gas in the presence of anhydrous aluminium chloride ($\mathrm{Al}{\mathrm{Cl}}_3$) or cuprous chloride ($\mathrm{CuCl}$) as a catalyst, benzaldehyde ($C_6{H}_5\mathrm{CHO}$) is formed.

This reaction is known as the Gattermann-Koch reaction. It is an electrophilic aromatic substitution reaction used for the formylation of aromatic rings. In this process, the mixture of $\mathrm{CO}$ and $\mathrm{HCl}$ acts in situ to generate formyl chloride ($\mathrm{HCOCl}$), which is unstable. Anhydrous $\mathrm{Al}{\mathrm{Cl}}_3$ facilitates the generation of the active electrophile, the formyl cation (${\mathrm{HCO}}^{+}$), which attacks the benzene ring to yield benzaldehyde.

The chemical equation representing the reaction is:
$C_6{H}_6+\mathrm{CO}+\mathrm{HCl}\stackrel{\text{Anhydrous}\mathrm{Al}{\mathrm{Cl}}_3}{\to }{C}_6{H}_5\mathrm{CHO}+\mathrm{HCl}$