Which word is used to introduce a non-essential clause, typically set off by commas (or dashes)?

Which word is used to introduce a non-essential clause, typically set off by commas (or dashes)?

• A. That
• B. Who
• C. Which
• D. What

Answer: (C)

Nonessential information adds extra detail that can be removed without changing what the sentence is about. That kind of detail is set off by commas or dashes. Which is the relative pronoun used to introduce these nonrestrictive (nonessential) clauses. For example, in The book, which won an award, was published in 1999, the clause after which adds extra detail about the book and can be removed without changing the main point. If you swapped in that instead—The book that won an award was published in 1999—the information becomes essential to identify which book is being talked about, so it wouldn’t be set off by commas. Who introduces clauses about people, not things; what introduces a noun clause rather than a relative clause. So which is the best choice for introducing a nonessential clause.

Nonessential information adds extra detail that can be removed without changing what the sentence is about. That kind of detail is set off by commas or dashes. Which is the relative pronoun used to introduce these nonrestrictive (nonessential) clauses. For example, in The book, which won an award, was published in 1999, the clause after which adds extra detail about the book and can be removed without changing the main point. If you swapped in that instead—The book that won an award was published in 1999—the information becomes essential to identify which book is being talked about, so it wouldn’t be set off by commas. Who introduces clauses about people, not things; what introduces a noun clause rather than a relative clause. So which is the best choice for introducing a nonessential clause.