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

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)

Nonessential (nonrestrictive) relative clauses add extra information and are set off by commas. In the sentence with the clause “who lives in Boston,” that extra detail describes the sister but isn’t needed to identify which sister is meant, so it should be enclosed by commas on both sides: My sister, who lives in Boston, is visiting. Without the surrounding commas, the clause would be read as essential, implying there could be multiple sisters and we’re identifying the one who lives in Boston. Leaving out the final comma or omitting commas entirely changes the meaning or breaks the punctuation rule. So the sentence with commas around the nonessential clause is the properly punctuated choice.