Question 22: Mark the letter A, B, C, or D on your answer sheet to indicate the undelined part that needs correction in each of the following questions: Not until I was on my way to the airport that I realized I had left my passport at home.
that I realized => did I realize
Structure Not until + normal Question + inverted Question: Until
In the latter part we defect with the auxiliary verb so realized in the past we use the auxiliary did did and invert before the subject of the clause.
It wasn’t until I was on my way to the airport that I realized that I had left my passport at home.