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 formed when chlorobenzene reacts with a molecule of chlorine?

1,4-Dichlorobenzene 

The correct answer is option 3. 1,4-Dichlorobenzene.

When chlorobenzene reacts with chlorine in the presence of a catalyst like iron(III) chloride \((FeCl_3)\), the major product is 1,4-Dichlorobenzene. Chlorine is an ortho/para-directing group due to its electron-donating resonance effect, despite being a deactivating group overall because of its electron-withdrawing inductive effect. The para position is favored over the ortho position due to reduced steric hindrance.

So, the correct answer is 1,4-Dichlorobenzene.