Which choice best describes the following sentence?

everyone should join a service club, it is a great way to improve the world.
it is a complete and correct sentence.
it features a comma splice.
it is a fused sentence.
it is a rambling sentence.

It features a comma splice.

Commas do not separate sentences - UNLESS AND ONLY IF there is a coordinating conjunction* after it.

* and, but, yet, so, or, for, nor

The answer to your question is "It features a comma splice."

A. To establish the main idea of the essay.

Explanation:

The purpose of the introduction in an essay is to meet the reader with the main idea. In the introduction, the author sets the main idea of the essay so readers can see in which direction is the essay going.

The introduction includes the main idea, how it will be developed ( but in the introduction there is no explaining the whole process of the main idea and all the ways of development) and the basic background information which is helpful to the reader to meet up with the topic.