Final answer:
The period of 900 BCE to 400 BCE in Mexico refers to the Olmec civilization, famous for their colossal stone sculptures, ball courts, and cultural contributions, including early developments in mathematics and architecture.
Step-by-step explanation:
The time period of 900 BCE to 400 BCE in Mexico is closely associated with the Olmec civilization, which is considered Mesoamerica's earliest complex society. This era predates the classic achievements of later Mesoamerican civilizations such as the Maya and the Aztec.
The Olmecs are known for their massive stone head sculptures, ball courts, and significant contributions to the establishment of social and cultural patterns in the region. Predominantly located in the south-central regions of Mexico, the Olmecs laid the foundation for future advancements in mathematics, architecture, and the creation of intricate calendars among Mesoamerican cultures.