The best answer is B.
While the countries of North Africa may be geographically separated from sub-Saharan Africa by the Sahara Desert, they are not considered isolated from the rest of Africa in a way that would prevent cultural, historical, and economic interactions. The other options (A, C, and D) reflect significant reasons why the countries of North Africa are studied together, including shared history (A), cultural influences from Europe (C), and a shared religion (D).