"When did the fire start?"
Translation:מתי התחילה השריפה?
It's hard to explain. אש is used when the fire is under control, and שריפה is used when it isn't and usually you don't want it to happen.. I'll give you an example: יש שריפה במטבח חם לי בגלל האש
I wrote it exactly as it appears in the answer, and they marked it wrong!