Final answer:
Trade networks were the primary factor in increasing wealth and power in West, East, and South African kingdoms due to the critical role of the gold, salt, and slave trades in the expansion of trade routes and economic prosperity.
Step-by-step explanation:
The factor that contributed the most to the increasing wealth and power of kingdoms in West, East, and South Africa was B) Trade networks. The bustling trade in gold, salt, and slaves significantly boosted the economies of various African civilizations. Through taxing trade routes, city-states grew into wealthy kingdoms and empires, and trade allowed even the most ecologically constrained societies to thrive. While military conquests and religious developments did have their impacts on the structure and culture of African societies, it was trade that predominantly drove the expansion and prosperity of these states. The gold trade, in particular, spurred the construction of expansive networks that not only facilitated the exchange of commodities but also cultural and technological ideas across the Saharan, Mediterranean, and Indian Ocean trade routes.