which of these sentences is an independent clause?

1. while marvin plays air guitar in the back seat.

2. unless she can get the gum out of her hair in time for the big dance.

3. the superhero is afraid of heights.

4. until is over.

choice 3. The superhero is afraid of heights

The other choices are sentences that are incomplete because they need more structue to them. For example, sentence 1 implies that something else is going on during Marvin's guitar time. The other sentences have cliffhangers as well and they arent fully independent or on their own. Sentence 3 expesses a single complete thought without introducing some other related or tangential thought needed to complete that idea.