Which of the following is considered to be the cause of decolonization of africa? a. the use of civil disobedience b. peaceful transfer of power c. war between the colonies and the european powers d. african nationalism combined with the changing attitudes towards imperialism and european countries

Sometimes some savvy test-takers say that the longest answer maybe is the one that is the best answer, because it's making sure it says all it's supposed to say.  I wouldn't go with that theory all the time, but in this case, the longest answer (D) is in fact the best answer.

Coming out of the period of the World Wars, there was a prevailing attitude among the Allies that people should be allowed self-determination of their nations and governments.  In other words, they were leaving behind attitudes favoring colonialism and imperialism, believing all nations should be able to determine their own destinies.  Coupled with that were rising nationalistic attitudes in African nations that had been dominated by European imperialism.  Those two factors started the trend toward decolonization.