At which of the following locations would you be most likely to find a high population density? within the sahel near the rainforest along the banks of the niger river in the middle of the sahara desert

You would be mostlikely to find a high population density "along the banks of
the Niger River," since rivers bring water, which is a crucial element of human survival. 

In the late fifteenth century, a major world power that established colonies in the era of old imperialism was france or germany or spain. their colonial activity was mainly limited to america or africa or asia.