The essential difference between an AC generator and a DC generator is that 

Option D

Slip rings are used in AC generators. A slip ring is an electromechanical device that permits power and electrical signals to be transmitted from a stationary structure to a revolving structure. A commutator is used in a DC generator. The act of converting generated alternating current in the armature winding of a DC machine into direct current after passing through the commutator and fixed brushes is known as commutation in DC machines or more particularly commutation in DC generators.