a

She is reading a book.

【Translation】She is reading a book. 【Explanation】The present progressive tense is structured as [Subject (She) + be-verb (is) + verb-ing (reading) + Object (a book)]. The correct order is 'She is reading a book.' The 3rd word (★) is 'a'.

【和訳】彼女は本を読んでいます。 【解説】現在進行形（〜しているところだ）は【主語 (She) ＋ be動詞 (is) ＋ 動詞-ing (reading) ＋ 目的語 (a book)】の語順になります。正しい文は「She is reading a book.」となり、3番目の空欄（★）には「a」が入ります。