Correct Answer - `FeCl_(3)`
The ethylation reaction of benzene involves the addition of an ethyl group on the benzene ring. Such a reaction is called a Friedel-Craft alkylation reaction. This reaction takes place in the presence of a Lewis acid.Any Lewis acid like anhydrous `FeCl_3, SnCl_4, BF_3` etc. can be used during the ethylation of benzene.
