A: It was an act of enslaving conquered people. No one captured could be happy to serve as a slave. A can't be the answer.
D: A better way is to set up trade with such a colony. Enslavement is not a good answer. D is incorrect.
C: It wasn't the intention of Spain to make the colonies rich. The intention was to make Spain rich. This is not the best answer.
B: I think you are intended to pick B..