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

• A. My sister, who lives in Boston, is visiting.
• B. My sister who lives in Boston is visiting.
• C. My sister, who lives in Boston is visiting.
• D. My sister who lives in Boston is visiting.

Answer: (A)

The main idea tested here is using commas to set off a nonessential (nonrestrictive) clause. This kind of clause adds extra information about the noun but isn’t needed to identify which noun is being talked about, so it’s separated by commas. In the sentence that fits, the phrase “who lives in Boston” provides additional detail about My sister and can be removed without changing the main meaning, which is that someone is visiting. Therefore, it should be enclosed with commas on both sides: My sister, who lives in Boston, is visiting. The other versions either skip a necessary comma or place it incorrectly, turning the clause into essential information or creating a punctuation error, which changes meaning or makes the sentence ungrammatical.