The petrol warning light . . . . . . . . for half an hour before we . . . . . . . . a petrol station.

Answer: Option A

The sentence describes an action that was happening continuously in the past until another action occurred.
The past perfect continuous tense (had been flashing) is used to indicate that the petrol warning light was flashing for a duration of time before the second action happened.
The simple past tense (reached) is used for the action that occurred after the continuous action.
Therefore, the correct sentence is: The petrol warning light had been flashing for half an hour before we reached a petrol station.