Answer: Because a mother kangaroo has a built-in baby carriage
Step-by-step explanation:
An adverb clause in a sentence is a group of words or a phrase that does the work of an adverb which is to provide descriptions for verbs and adjectives amongst others.
For a group of words functioning as an adverb to be an Adverb clause, they must have a subject and a verb otherwise they'll be an adverb phrase. Also, adverb clauses usually have a comma after them in a sentence.
In the options listed, the adverb clause would have to be, Because a mother kangaroo has a built-in baby carriage, because it is functioning as an adverb and has a subject being the Mother Kangaroo as well as a verb being has.