The correct options are
A The coast of the Gulf of Guinea
D The highlands of Madagascar
The highest population densities in Africa are observed along the coast of the Gulf of Guinea, in the lower valley of River Nile (Egypt), in the highlands of East Africa and Madagascar, along the northern coast and in the urban and mining areas of South Africa, Zimbabwe, and Zaire. The lowest population density is observed in the Sahara Desert, Liberia, and Algeria.