Final answer:
In 'Percy Jackson & the Olympians: The Lightning Thief,' the stolen lightning bolt must be returned by the summer solstice to prevent chaos and war among the gods.
Step-by-step explanation:
The deadline set for the return of the stolen item in Percy Jackson's story is the summer solstice. According to the plot of 'Percy Jackson & the Olympians: The Lightning Thief' by Rick Riordan, if the stolen lightning bolt is not returned to Mount Olympus by this date, it could potentially lead to chaos and war amongst the gods, thus affecting the entire world. The precise option that reflects this deadline from the provided choices is 'b) The deadline is the end of the summer.'
In Percy Jackson's story, the deadline that has been set for the return of the stolen item is the winter solstice. If the item is not returned by then, it will lead to chaos and war.