What country in saharan and southern africa has the highest population?
a. south africa
b. nigeria
c. angola
d. congo

South Africa= around 55,000,000 of total population (based on 2015 data)
Nigeria= around 185,000,000 of total population  (2015 data)
Angola = around 25,000,000 of total population    (2014 data) 
Congo= around 82,000,000 of total population   (2015 data) 
Given that there's not an high population growth or birth rate or something like that, I'd say Nigeria