best

母は家族の中で一番料理が上手です。 My mother ＿＿ ＿＿ ★ ＿＿ cook in my family.

The correct order is 'My mother is the best cook in my family.'. The 3rd word (★) is 'best'.

正しい語順は My mother is the best cook in my family. (the + 最上級 best + 名詞) となります。3番目の空白（★）には best が入ります。