The most accurate answer to how the partitioning of Africa by European nations caused lasting civil and regional conflicts in modern Africa is:
B. Partitions ignored existing ethnic and political boundaries between African nations.
This option highlights how the arbitrary borders drawn by European powers during the colonial era often neglected the complex social, ethnic, and political landscapes of the continent. By forcing diverse groups into single political entities without regard for their traditional boundaries or governance systems, the partitioning set the stage for ongoing conflict and instability in many regions of Africa.