The correct answer is A. a 365-day calendar.
Both the Aztecs and the Maya developed a calendar system that included a 365-day solar calendar. While the Maya had an advanced understanding of the calendar with the Tzolk'in (260 days) and the Haab' (365 days), the Aztecs used a similar system as part of their calendrical practices.
The other options are not accurate for both cultures:
- B. Counting knots called quipu was developed by the Incas, not the Aztecs or Maya.
- C. The Maya primarily used a base 20 number system, while the Aztecs used a base 20 system as well, but it wasn't distinctly developed in the same way by both.
- D. The mathematical concept of zero was prominently used by the Maya, but it is less clear how it was employed by the Aztecs.
Hence, option A is the most appropriate choice.