Answer:
C (or B)
Step-by-step explanation:
Although all of the options probably contributed to the Incas success, it seems as though C does not help them control the people. Splitting the empire into smaller regions would help them manage large amounts of people easier. The cacao currency would help unify the empire, making the everyday exchange of goods easy, which motivates people to join/remain in the empire. An extensive road system would allow messages to get to people, but the empire does not need this to be large. People providing labor would keep the empire large and plentiful with supplies. I personally think B, but I think it is best for you to decide overall.