B. and E. marked the downfall of the Inca empire, for obvious reasons. Disease and civil war would obviously kill the empire, and taking an important leader would also mark great chaos within the empire. A. and C. are not to be ignored however, considering that maintaining peaceful relationships with the Spaniards only brought death and chaos to the Aztecs, all from the Spaniards.
The answer is: The sea
Back then, the techology that exist to travel by sea was not as developed as it is today. If an empire decided to travel through the sea in order to do some conquests, that empire will risk several things, such as:
- Loss in directions
- The ships could be swallowed by harsh wheather in the middle of the sea
- It need a lot of resources to maintain the armies during the travel
