Which sentence uses a nonessential clause correctly with punctuation?

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

Answer: (A)

Nonessential (nonrestrictive) clauses add information that isn’t needed to identify the subject and should be set off with commas. The phrase who lives in Seattle is such a clause here, because it gives extra detail about my sister rather than defining which sister is being talked about. Proper punctuation uses a pair of commas to enclose that extra information, so the sentence reads smoothly: My sister, who lives in Seattle, is visiting. If you drop the commas or misplace them, you change how the clause functions or create a punctuation error: without commas, the clause would be treated as essential and could imply there are multiple sisters; with the comma missing after Seattle, the nonessential information isn’t properly separated; and placing a comma after the clause doesn’t fit how nonessential clauses are normally punctuated. So the sentence with the two commas around the clause is the correct choice.