The introduction of an alkyl group in benzene takes place in the presence of AlCl3 and the reagent is:

Explanation

The introduction of an alkyl group (an alkyl substitution) into benzene takes place through a Friedel-Crafts alkylation reaction. The reagent used in this reaction is the alkyl halide (alkyl halogenide) or alkyl chloride (alkyl chlorides are a specific type of alkyl halide). The reaction is catalyzed by a Lewis acid, typically aluminum chloride (AlCl3) or ferric chloride (FeCl3). The general reaction scheme for Friedel-Crafts alkylation is as follows: Benzene + R-X (alkyl halide) → Alkylbenzene + HX (hydrogen halide).Where R is the alkyl group (e.g., methyl, ethyl, propyl, etc.), X is the halogen (e.g., Cl, Br, I), and HX is the hydrogen halide formed as a byproduct. Therefore, option B is correct.