A subordinate clause is a group of words that has both a subject and a verb but cannot stand alone as a sentence.
Of the given options, we see that
I like hotdogs. - Sentence
The little baby cries all night. - Sentence
The dog crawls toward the door for food. - Sentence
Until the sun goes down - This is not complete and is depending on other part to get complete or convey a meaning.
Hence the answer is
C. Until the sun goes down
