C. Saudi Arabia is the answer.
Sub-Saharan Africa's equatorial area is known as Equatorial Africa. There are 13-14 nations, so I'm guessing you want Ethiopia, Rwanda, Kenya, and Somalia, which are the most well-known. Savannas in the north and south, as well as mountains and tropical rain forests, characterize the landscape.
One way in which the study of populations ( human populations - I assume) affects us is that they can help predict the population size in the future and prepare us for it. For example, if we see that the population is decreasing, a country might want to consider supporting child-bearing or immigration to avoid the problem of too many people receiving pensions but not paying taxes.
