Why not אש?
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
Is it incorrect to say מתי השריפה התחילה?
Then why is it marked wrong!!!!
למה אין פה "את"
Because fire in this sentence is the subject, not the object. את is used only before the direct object.
Matai hitchila hasrefa?
Same problem here
I wrote it exactly as it appears in the answer, and they marked it wrong!