Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom. Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions. Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.

What is the major product when chlorobenzene reacts with acetyl chloride? 

4-Chloroacetophenone

The correct answer is option 3. 4-Chloroacetophenone.

When chlorobenzene reacts with acetyl chloride in the presence of a Lewis acid catalyst like anhydrous aluminum chloride \((AlCl_3)\), an electrophilic aromatic substitution reaction occurs, specifically Friedel-Crafts acylation. The major product will be 4-Chloroacetophenone. The chlorine atom on the benzene ring is an electron-withdrawing group and directs incoming electrophiles to the ortho and para positions. However, due to steric hindrance at the ortho position, the para position is favored for the acylation reaction.

So, the correct answer is 4-Chloroacetophenone.