The answer to the question above would be that the sentence in which an adverb clause is underlined is the following one: Although people live in the desert, they are still limited by its harsh climate. That is, the correct option is C.
An adverb clause is a group of words that functions as an adverb in a sentence. As they are dependent clauses, they must have a subordinating conjunction to connect them to the rest of the sentence. In addition to this, they can be placed at the beginning, the middle, or end of the sentence. When placed at the beginning or in the middle, they require a comma to offset them from the rest of the sentence.