Right answer:
para
Para is a preposition. A preposition is a word (or group of words) which is used to show the way which other words are connected, that is, they are used to indicate a relationship between two words or phrases, between nouns, between nouns and verbs, and between different parts of a sentence. So para can be translated into for, by, on, through, because of, in exchange for, in order to, and several other prepositions and phrases. Here is used used to talk about goals and purposes. Therefore:
Para sentarnos, usamos un sillon