eating

父は朝食を食べたあと仕事に行きました。 My father went to work ＿＿ ＿＿ ★ ＿＿ breakfast.

The correct order is 'My father went to work after eating breakfast.'. The 3rd word (★) is 'eating'.

正しい語順は My father went to work after eating breakfast. (前置詞 after + 動名詞「〜したあとに」) となります。3番目の空白（★）には eating が入ります。