Which sentence correctly uses a comma to set off a nonessential clause?

• A. The book, which was on the table, is mine.
• B. The book which was on the table is mine.
• C. The book, which was on the table is mine.
• D. The book which was on the table, is mine.

Answer: (A)

Nonessential (nonrestrictive) clauses add extra information and are set off by commas. In this sentence, "which was on the table" provides additional details about the book but isn’t needed to identify which book is mine. Because of that, the clause should be enclosed by commas: The book, which was on the table, is mine. Removing the clause leaves a complete, sensible sentence: The book is mine.

The other forms don’t use the punctuation correctly for a nonessential clause. One version omits the closing comma after the clause, which makes the structure run together; another places a comma after the clause, which isn’t how nonessential clauses are punctuated; and another uses no commas around the clause at all, making it read as essential rather than nonessential. The correctly punctuated sentence clearly marks the extra information with two commas.