Which sentence demonstrates correct comma usage with a nonessential clause?

• 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 clauses add extra information about a noun and are set off by commas. In this sentence, the clause "who lives in Seattle" gives extra detail about the sister but doesn’t identify which sister is being talked about. The main idea remains that the sister is visiting, so the extra detail is optional and should be enclosed by a comma before the clause and after it: My sister, who lives in Seattle, is visiting.

Omitting the closing comma or placing the comma in the wrong spot changes the meaning or breaks the rule. If no commas surround the clause, as in My sister who lives in Seattle is visiting, the clause becomes essential, implying there may be more than one sister and we’re specifying which one. If the punctuation crisscrosses so the clause isn’t properly enclosed, the sentence sounds off and disrupts the flow. So the correct sentence uses commas on both sides of the nonessential clause, clearly signaling that the information about Seattle is extra.