Answer:
1 3 5
Step-by-step explanation:
Whenever I go to the movies, I buy popcorn.
I buy popcorn is the main clause. The subordinate clause is Whenever I go to the movies. It's the one you want.
2 is a complete sentence. I think you are after subordinate clauses.
3 A tornado tearing through town [destroyed the library]. This one counts.
4 [It didn't look cloudy], but it's likely to rain that day. These two are main clauses, they don't count.
5 After the end of the school year is a subordinate clause. It counts.