in

My brother is in his room.

【Translation】My brother is in his room. 【Explanation】Following the subject 'My brother', the structure is [be-verb (is) + Preposition of place (in) + Possessive (his) + Noun (room)]. The correct order is 'My brother is in his room.'. The third blank (★) should be 'in'.

【和訳】私の兄（または弟）は彼の部屋の中にいます。 【解説】主語「My brother」に続き、【be動詞 (is) ＋ 場所を表す前置詞 (in) ＋ 所有格 (his) ＋ 名詞 (room)】の語順になります。正しい文は「My brother is in his room.」となり、3番目の空欄（★）には「in」が入ります。