**Core Concept:** This question tests the understanding of descriptive phrases, specifically how they modify nouns, and the difference between restrictive and non-restrictive modifiers. It also requires differentiating between adjectival and adverbial functions. **Step-by-step analysis:** 1. **Identify the core sentence:** The main sentence structure is 'The mountains stood as silent sentinels'. 2. **Analyze the phrase:** 'their peaks shrouded in mist' describes the mountains. This phrase adds detail about the mountains, it is nonessential, and is set off by a comma. 3. **Determine the phrase type:** It is not an independent clause; instead, it is a modifying phrase with a participle ('shrouded'). Therefore it acts as an adjective. 4. **Evaluate the options:** * **A:** An independent clause is able to stand on its own as a sentence, the phrase in question does not contain a verb. Thus, A is incorrect. * **B:** This phrase acts as a modifier not a subject, since the subject is 'mountains'. Thus, B is incorrect. * **C:** The phrase is non-restrictive adjectival because it adds descriptive detail and does not essential to the meaning of the sentence, and modifies mountains. Thus, C is correct. * **D:** It doesn't describe how the mountains stood; rather it describes a characteristic of the mountains, thus it isn’t an adverbial phrase. Thus, D is incorrect. **Key Points:** * Non-restrictive modifiers add extra, non-essential information and are set off by commas. * Adjectival phrases describe nouns or pronouns, while adverbial phrases describe verbs, adjectives, or other adverbs. * Participial phrases can function as adjectives, adding descriptive detail. **Error Prevention:** Option A could be tempting for someone who doesn’t understand that a clause must have a subject and verb. Option B could attract someone who struggles with identifying parts of speech. Option D could be appealing since the word 'stood' is a verb and might prompt the student to analyze it with an adverbial phrase.