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.