some

私の妹は牛乳を買うためにスーパーへ行きました。 My sister went to the supermarket ＿＿ ＿＿ ★ ＿＿ milk.

The correct order is 'My sister went to the supermarket to buy some milk.'. The 3rd word (★) is 'some'.

正しい語順は My sister went to the supermarket to buy some milk. (不定詞の副詞的用法「〜するために」 + buy + some + 名詞) となります。3番目の空白（★）には some が入ります。