Question

Which motivation was the most important for European colonization of Central Africa

Answer 1

Their reason was more economic

So the answer is "exploiting the colonies for profit"

Answer 2

The answer is to colonize Africa because of its rich resources.
Since European countries like England, France, and and Germany was under depression that time, they wanted to find colonies that can provide additional resources. They can use the resources they have gathered for their own empire or they can use it to trade to other countries for finance.