Answer: Adjective clause: that is sitting in the window.
Step-by-step explanation: a subordinate or dependent clause is a phrase that doesn't have complete meaning on its own. In the given sentence, the subordinate clause is "that is sitting in the window" (notice that if we remove it, the sentence still makes sense), and it is an adjective clause because it is modifying the noun "cat" (by giving additional information) which is also the subject of the sentence.