The best answer is B. The Tlaxcalans and the Totonac were eager to get out from under the self-imposed rule of the Aztec.
Cortés enlisted the Tlaxcalans and the Totonac because they had long been enemies of the Aztec Empire and were willing to ally with the Spanish forces to overthrow their common enemy. Their desire for freedom from Aztec dominance motivated their collaboration with Cortés.