Answer: D
Explanation: The Ghana Empire, properly known as Wagadou (as Ghana was the title of its ruler), was a West African empire located in the area occupied by present-day southeastern Mauritania and western Mali. Although complex societies based on trans-Saharan trade in salt and gold had existed in the region for centuries, the introduction of the camel to the western Sahara in the 3rd century CE, opened the way to great changes in the area that eventually became the Ghana Empire.
The Ghana Empire grew rich from increased trans-Saharan trade in gold and salt, allowing for larger urban centres to develop. With these commodities being transported and traded through Ghana, the Kingdom was able to become very wealthy by taxing the goods that came through the trade center. It also helped that Ghana had many trade routes that were well protected by its large military force as this encouraged other merchants to come to Ghana for trade.