Final answer:
The Mayan civilization around 900 CE predominantly declined due to a prolonged drought that led to environmental degradation and thereby affected their agriculture-dependent society.
Step-by-step explanation:
The decline of the Maya civilization around 900 CE has been attributed to various causes including drought, epidemics, invasions, and internal conflicts. However, one significant factor that contributed to their decline was environmental degradation--particularly the drought that lasted nearly two centuries which affected their agriculture-based society.
This period of prolonged drought made it difficult to sustain the large populations in the Mayan city-states and is thought to be one of the key reasons the Maya civilization experienced decline and eventually abandoned their large population centers, not to be confused with the later collapses during the times of European invasions.