The Congo Basin is the largest drainage basin in Africa, enclosing a vast rainforest ecosystem rich in biodiversity. It`s a crucial part of the Earth`s entire ecological system.

Option 1, Cameroon, is correct. The Congo Basin covers parts of nine African countries, including a significant part of Cameroon.

Option 2, Nigeria, is not part of the Congo Basin. It lies to the west of the Basin and is mostly drained by the Niger River instead.

Option 3, South Sudan, lies to the east of the Congo Basin. Its water drainage is mainly into the Nile River system, not the Congo River system.

Option 4, Uganda, is also not part of Congo Basin. Uganda is outside the geographical zone of the basin and its water drainage system is largely associated with the Nile River, not the Congo River.

In conclusion, the only country on this list that includes parts of the Congo Basin is Cameroon.