The three major west african empires increased their wealth by
trading gold and precious jewels.
taxing all trade in their region.
restricting trade to only high-level merchants.
pooling their trade earnings into one central account.

The correct answer is taxing all trade in their region

A huge amount of traders went to the African empires because of easy access to things like gold and jewels, or even more importantly salt which was a precious thing at that time. The government taxed all trading and since everyone wanted their goods the government became extremely wealthy and nobody could compete with them.

Ialso think the answer to this is korean