Non-defining relative clauses provide extra information that isn't crucial to the sentence's main meaning and are always set off by commas. The relative pronoun 'which' is commonly used in non-defining clauses. * **Core Concept:** Understanding the use of relative pronouns in non-defining clauses. * **Solution Analysis:** Option C demonstrates the correct usage. * **Detailed Option Analysis:** * **A:** Incorrect. 'That' cannot be used in non-defining clauses. * **B:** Incorrect. Lacks the commas needed to set off non-defining clause * **C:** Correct. 'Which' is correctly used and set off by commas. * **D:** Incorrect. 'That' cannot introduce non-defining clause * **Common Mistakes:** Confusing 'that' and 'which', or not using commas correctly in non-defining clauses.