Answer:
C. Adverbial clause
Explanation:
The main difference between a clause and a phrase is that while the first one always has at least a subject and a verb, a phrase does not. So, according to this definition, we can discard option A, B, and D.
Therefore, option C is the correct answer because an Adverbial clause includes a verb and acts as an adverb, meaning that it modifies or adds more detail to an adjective, a verb or another adverb. This type of clauses often answers questions such as "Why?", "How?", "When?", "Where?" and "In what circumstances?".
Example (Adverbial clause underlined): <u><em>Before eating her lunch</em></u><em>, she washed her hands.</em>
(When did she wash her hands? Answer: Before eating her lunch. This clause adds information about WHEN an action took place, and it modifies the verb "washed").