It seems that you have missed the given choices of the question above; but anyway, here is the correct answer. The primary basis of Aksum's wealth is that, they controlled extensive trade routes that connected many cultures.
Here are the given choices:
A They discovered rich gold deposits within the boundaries of the kingdom.
B They controlled extensive trade routes that connected many cultures.
C They exported thousands of slaves to Europe.
D They conquered Egypt and assimilated all of Egypt's wealth.
