In sub-saharan africa, female genital mutilation is
a. unheard of in central africa and namibia.
b. uncommon in nigeria.
c. rare in mali.
d. most common in northern countries of the region.

Female Genital Mutilation (FGM) is common in Central Africa, Nigeria and Mali, which means that we can reject answers A,B and C (it's at least 20% in all of them and over 80% in Mali).

The correct answer is that it is most common in northern countries of the region.  - for example in Mali and Sudan.