Pivot mean the central point, pin, or shaft on which a mechanism turns or oscillates.
First you need to know what a sentence fragment is and also what a complete subject and predict is. That should help you.
'Before they ate a snack' is the correct answer, it is the adverbial clause of time. A cannot be correct because that is an independent clause, and adverb clause has to be dependent. B isn't a clause at all.
