Final answer:
The most notable expansion of the Aztec Templo Mayor was during 1440-1481 CE under Montezuma I, characterized by elaborate sculptures and carvings.
Step-by-step explanation:
The most spectacular expansion of the Aztec Templo Mayor occurred during the reign of Montezuma I between 1440 and 1481 CE. The Aztec Empire was at the height of its civilization during this time, which allowed for significant resources and labor to be devoted to the temple's construction.
This period included elaborate sculptures and ornate carvings such as the undulating serpents that adorned the temple's stairs and the giant sculpted frogs at the shrine's center.