Answer:
C. an adverbial clause
Explanation:
An adverbial clause is a group of words that have at least a subject and a verb, and whose function is to act as an adverb, that is to say, to modify or to add 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?".
So, for there were stars out is an adverbial clause because it gives further information or detail to the action (verb) of not knowing where the snow came from, and it also answers the question "Why": Why don't you know where the snow came from? Answer: for there were stars out.
