Which of the Following Is NOT One of the Three

Which of the following is NOT one of the three most important uses of the comma?

A) to separate two main clauses that have no conjunction between them
B) to enclose or separate nonessential words, phrases, or clauses that come within or after a main clause
C) to separate a main clause from introductory word(s)
D) to separate two main clauses that have a coordinating conjunction between them