Directions: Find the part of the sentence which has an error. If there is no error select (d) as your answer.

Although we reached his house on time (a) / he was left (b) / for the airport. (c) / No error (d)

Case of improper verb form usage. Whenever two actions in the past happen one after the other, we use past perfect tense and insert 'had' before the action that actually takes place earlier in time. As is clear in the given sentence that when 'we' reached there 'he' was gone,  so we should change 'he was left' into 'he had left'.