4. Cavern
In the excerpt it is clear that a grotto is something you can enter through an opening. The only other word in the excerpt that you can enter is a cavern. For this reason, we can eliminate air, opening, pure, and atmosphere. If you do not know what mephitic means this could still be an option. However, if we look at how mephitic is used in the sentence, we know that it is an adjective that can describe how something smells. You can't walk into a description of a smell through an opening, so mephitic can also be eliminated.
