The response that best summarizes how the Aztec Empire was created is:
The Mexica started the Triple Alliance with two other groups in order to fight an oppressive kingdom in the Valley of Mexico.
This reflects the key historical moment when the Mexica (or Aztecs) allied with other city-states, specifically Texcoco and Tlacopan, to combat the more dominant Tepanec rulers, which was crucial in establishing their empire.