The correct answer is Option C. Relations between Kongo leaders and the Portuguese became strained.
Relations between Kongo leaders and the Portugese became strained as a result of Slave Raiding by the Portuagese.
In the 'age of exploration' and more precisely the European Colonisation of Africa, every colonial power was looking to establish a large trading base.
Portugese used to raid Kongo (Congo) to capture, kidnap and enslave localpeople and then sell them off in the slave trade business. These slaves were usually sold off in the new world to work on sugar, cotton and other plantations.
The Portugese established a profitable business out of African slavery but it started to create huge friction between them and the local leaders.