The correct response is: Their god Huitzilopochtli told them where to begin developing their new city on a spot he provided them.
The Mexica (also known as the Aztecs) believed that their god Huitzilopochtli guided them to the location for Tenochtitlan, where they eventually established their capital. According to their mythology, they were instructed to find an eagle perched on a cactus, which was the sign to settle in that place.