Final answer:
The increasing wealth and power of kingdoms in various parts of Africa was primarily due to the Trans-Saharan Trade, which connected African empires like Ghana and Mali to other parts of the old world, allowing them to trade gold, salt, and other commodities.
Step-by-step explanation:
The most substantial factor contributing to the increasing wealth and power of kingdoms in West, East, Central, and South Africa was a) Trans-Saharan Trade. This trade route was instrumental in the distribution of commodities like gold, ivory, salt, silk, horses, and enslaved people. The empires of Ghana and Mali in West Africa utilized a network that connected them to the Mediterranean world, Europe, the Near East, Southwest Asia, and even further reaches. Similarly, the kingdoms in Southern Africa, such as Mapungubwe and Great Zimbabwe, became powerful by controlling the regional trade and connecting their economies with that of the Swahili coastal civilization.
This network involved waterways such as the Limpopo and Zambezi Rivers and stretched as far as the Indian Ocean, linking the interior of Southern Africa with Arabia and Asia. The wealth generated from these trade relations allowed for the minting of European coins from West African gold and spurred the growth of expansive trade routes across the Sahara. This economic power translated into political might, enabling these states to flourish and expand their influence across the continent, supported by additional factors such as the Arab invasion and the Bantu migration, which fostered cultural and economic unification.