How can you identify a non-essential clause?

The correct answer highlights that a non-essential clause can be removed from the sentence without altering its overall meaning. Non-essential clauses add extra information but are not crucial for the sentence to retain its main message. For example, in the sentence "The book, which was published last year, is on the table," the phrase "which was published last year" provides additional details but can be omitted without affecting the understanding of the core idea that "The book is on the table."

In contrast, essential clauses are necessary for identifying or clarifying the subject they modify. The other options present characteristics that do not accurately define a non-essential clause: a non-essential clause indeed cannot stand alone as a complete sentence, but this aspect applies to both essential and non-essential clauses. Saying that it always begins with a coordinating conjunction is not correct, as non-essential clauses usually start with subordinating conjunctions like "which" or "that." Lastly, a non-essential clause is, by definition, not always essential for the complete meaning of the sentence.