Answer:
2. an adverbial clause modifying the verb are keeping
Step-by-step explanation:
Adverbial clauses usually begin with a subordinating conjunction like since, after and before, and they contain a subject and a verb. Therefore, the sentence given is a complex sentence, consisting of a dependent or subordinate clause (Since they already have an older cat) and an independent one (they are keeping the kitten in a separate room for a few days).