The best explanation for why empires, including the Aztec Empire, often sought to expand their territory is:
They wanted not only to show their power, but also to acquire important resources.
This response captures the dual motives of expansion: the display of power through conquest and the practical need for resources such as land, wealth, and labor, which were crucial for the empire's sustainability and growth. While the other options present valid aspects of imperial behavior, the combination of power and resource acquisition is a primary driving force behind many empires' expansionist policies.