The correct answer is option 4, Rwanda.

To understand why Rwanda is located south of the equator, we need to first understand what the equator is. The equator is an imaginary line that divides the Earth into Northern and Southern Hemispheres. Countries that are located south of the equator are considered to be in the Southern Hemisphere.

Option 1, Cameroon, is located in Central Africa and is situated just north of the equator. Therefore, it is not south of the equator.

Option 2, Sudan, is located in Northeast Africa and is also situated just north of the equator. Hence, it is not south of the equator.

Option 3, Nigeria, is located in West Africa and is positioned a bit north of the equator. Therefore, it is not south of the equator.

Finally, option 4, Rwanda, is a landlocked country in East Africa. It is located just south of the equator, making it the correct answer to the question.

In conclusion, Rwanda is the only country among the options listed that is located south of the equator.