Which region is most closely associated with
the expansion of the sahel and overgrazing in
the savanna regions?
(1) south america (3) africa
(2) china (4) southeast asia

The region that is most closely associated with the expansion of the Sahel is (3) Africa. 

The correct answer is Africa because the Sahel is a region that can be found only in Africa and because the syptoms you've described in your question are things that have been happening in the Sahel region for quite some time now. If nothing will be done against them, there will be drastic increase of sand in Africa over the future years.