The correct answer is A. East African mountainous region made it difficult to protect itself from invaders.
East Africa is constituted, for the most part, by high altitude mountains, massifs, coastal plains and a large tectonic pit that covers the center of this region.
In East Africa the highest mountains of the continent, Kilimanjaro (5895 m) located in the northeast of Tanzania, and Mount Kenya (5199 m) in Kenya, are located.
This geographical situation, with sea access to the east and mountains to the west, has historically functioned as a natural barrier that greatly hindered the defense of its inhabitants to face external invasions that came from the Indian Ocean.