The correct choice is Option 3: 'Because keeping pets is prohibited in the apartment.' The passage explicitly states: 'でも、私の住んでいるアパートは、ペットを飼ってはいけないことになっています' (However, the apartment where I live is designated as a place where you must not keep pets). The pattern '〜てはいけないことになっている' indicates a formal rule or regulation. Analysis of incorrect choices: Option 1 is wrong because the author 'wants to keep a dog' ('犬を飼いたいです'). Option 2 is wrong because although the author wishes to move to a 'spacious house' ('広い家') in the future, the direct present reason they cannot keep a dog is the apartment rule, not the size. Option 4 is not mentioned.