than

この本はあの本よりも簡単です。 This book ＿＿ ＿＿ ★ ＿＿ that one.

The correct order is 'This book is easier than that one.'. The 3rd word (★) is 'than'.

正しい語順は This book is easier than that one. (形容詞の比較級 + than「〜よりも〜」) となります。3番目の空白（★）には than が入ります。