People in southern africa would be most affected by a) the flooding of the nile river. b) a decrease in trans-saharan trade. c) the spread of the kalahari desert. d) fish shortages in the indian ocean.

They'd be MOST affected by whichever answer choice would make climate change there even worse because in South Africa, the climate is already really bad.
Based on that I'd say THE ANSWER IS
C) the spread of the Kalahari Desert

The answer is all about the number 4