Question

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.

Answer 1

taxing all trade in their region.

They controlled trans-Saharan trade routes and gained wealth by levying taxes and tribute on caravans and merchants (and by monopolizing important goods like gold and salt).