The reasons for the fall of the Inca Empire primarily include disease, civil war, and Spanish conquest.
- Disease: European diseases, particularly smallpox, devastated the Inca population, who had no immunity to these illnesses.
- Civil War: The Inca Empire was also weakened by internal conflict, notably a civil war between factions led by Atahualpa and Huáscar shortly before the Spanish arrived.
- Spanish Conquest: The arrival of Spanish conquistadors, led by Francisco Pizarro, directly contributed to the downfall of the Empire through military conquest and exploitation of the disarray caused by disease and civil strife.
Thus, the correct answer from the options you provided is: disease, civil war, Spanish conquest.