Choose the sentence that correctly uses a nonrestrictive clause with commas:

• A. The author who wrote the book won an award.
• B. The author, who wrote the book, won an award.
• C. The author, who wrote the book won an award.
• D. The author who wrote the book, won an award.

Answer: (B)

The idea being tested is using a nonrestrictive relative clause with commas. A nonrestrictive clause adds extra information about the noun but doesn’t change which noun you’re talking about, so it’s set off by commas. Here, “who wrote the book” adds extra detail about the author, but it isn’t needed to identify which author is being referred to. Placing it between two commas makes it a parenthetical aside: The author, who wrote the book, won an award.

That’s why this sentence is the best choice: it clearly marks the extra information with commas, showing that the main point is that the author won an award, while the clause about writing the book is additional detail.

The other forms either leave out the necessary commas, making the clause restrictive (essential to identifying the author) or misplace punctuation so that the relative clause and the main clause aren’t cleanly separated.