Final answer:
The European expansion into Africa, mainly driven by economic interests in valuable natural resources and trade expansion, serves as the main reason behind Europe's increased presence on the continent, rather than exploration, cooperation in Christianity, or agreements with African leaders.
Step-by-step explanation:
The main reason why European powers expanded their presence in Africa was closely tied to economic benefits driven by the quest for resources and trade. This expansion was known as imperialism, where powerful countries sought to extend their authority beyond their own borders, often to acquire raw materials and open new trade markets. While some might believe that exploring African rivers and studying flora and fauna (A), spreading Christianity (B), or making agreements with local African leaders to work and trade with Arab traders (D) were the main reasons, evidence suggests otherwise.
European powers, such as the British, were particularly drawn to regions in Africa that held valuable natural resources, like gold and diamonds. These resources were subsequently sent back to Europe, aiding in the wealth and prosperity of the upper class and prompting further territorial expansion to secure such valuable commodities. For example, the British conquest of South Africa and the defeat of the Zulu Nation highlighted the great lengths to which European powers would go to dominate areas rich in resources, employing advanced technology in both weaponry and medicine to achieve their goals.
The desire to grow wealthy was a primary motivator for countries like Portugal and Spain to explore new territories, in addition to religious motivations like the spread of Christianity. The latter was a significant motive but secondary to the primary economic drivers. Hence, the most accurate answer to the question at hand is C: To expand their trade in Africa, the Indian Ocean, and India.