The Aztecs unified
A. in part because of famine. In a period of extreme hardship, communities came together to split labor obligations and share food supplies.
B. as a result of the Triple Alliance in 1431 CE. With the Triple Alliance, the Mexica achieved independence from their Tepanec patrons and aligned themselves with people in other outlying towns.
C. for the first time in the early 1500s in order to fight against Spanish conquest. After realizing their vulnerability to Spanish guns and steel, the Aztecs built an empire to defend themselves from European conquest.
D. after a military victory over the Olmec in 1291 CE. The military might of the Aztecs allowed them to easily defeat the Olmec in one major battle and prompted their unification.