Answer:
C. who
Explanation:
Adverb clauses' function is to modify or to add more detail to an adjective, a verb or another adverb, so it often answer questions like "Why?", "How?", "When?", "Where?" and "In what circumstances?." But since these clauses do not modify nouns, it can not answer to "Who?" questions or be introduced by "Who." Here are some examples of sentences containing adverb clauses (underlined):
After drinking that much alcohol, I felt sick.
The adverb clause modifies the verb "felt" and it answers the "when?" question: When did you feel sick? Answer: After drinking that much alcohol.
When Eli comes home, we always watch series all day long.
The adverb clause modifies the verb "watch" and it also answers the "when?" question: When do you watch series all day long? Answer: When Eli comes home.
Your shoes are in the living room, where I told you not to put them!
The adverb clause answers the "where?" question: Where are your shoes? Answer: Where I told you not to put them!
