The essential difference between an AC generator and a DC generator is that:
$(a)$. AC generator has an electromagnet while a DC generator has a permanent magnet.
$(b)$. DC generator will generate a higher voltage.
$(c)$. AC generator will generate a higher voltage.
$(d)$. AC generator has slip rings while the DC generator has a commutator.

Correct answer: $(d)$. AC generator has slip rings while the DC generator has a commutator.

The main difference between an AC generator and a DC generator:

The difference between an AC and a DC motor can be made as follows.

AC generator:

An electromechanical energy conversion device that converts mechanical energy into AC electrical energy is called an AC generator. AC generator has two rings and these two rings in the AC generator are called slip rings. These slip rings are used to change the direction of the current. Thus with the help of these two slip rings an AC current is produced by an AC generator. 

DC generator:

An electromechanical energy conversion device that converts mechanical energy into DC electrical energy is called a DC generator. DC generator has two half rings and these two half rings are called a commutator. The commutator keeps the direction of the current the same. Thus a DC current is produced by a DC generator. 

So, option $(d)$ is correct.