Answer:
C. Relations between Kongo leaders and the Portuguese became strained.
Step-by-step explanation:
Portuguese used to raid Kongo (Congo) to capture, kidnap and enslave local people 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 Portuguese established a profitable business out of African slavery but it started to create huge friction between them and the local leaders.