Concept:The Kanem-Borno Empire grew powerful by controlling key commercial routes across the Sahara Desert.
Explanation:The empire was strategically located around Lake Chad, linking Central Africa to North Africa.
It controlled the trans-Saharan trade routes that carried valuable commodities such as gold, salt, livestock, and slaves.
This control generated substantial wealth through taxes and tolls on all goods passing through its territory.
The resulting prosperity financed a strong army and expanded the political influence of the king.
It also allowed the empire to build diplomatic relationships with powerful North African states, which increased its prestige.
Agricultural surplus existed, but it was not the decisive factor behind the empire's rise in the 9th century.
Similarly, conquest of neighboring empires was an effect of its economic strength, not the primary cause of its rise.
The Ottoman diplomatic alliance only came much later, in the 16th century, so it cannot explain the 9th-century rise.
Answer:D. Control of trans-Saharan trade routes.