Answer:
C. To resist conquest by the Persians
.
Explanation:
Only option C is correct as during the Greco-Persian war these two city-states were practically obliged to form an alliance as Persians attacked them and made a great danger for whole Greece.
Although Sparta and Athens were never in good relations they had to put aside their differences to defeat the common enemy.
All other answers are wrong as they are not historically accurate.
