Option a is correct: The Congo Basin is a vast region in Central Africa, encompassing multiple countries. It is primarily characterized by the Congo Rainforest, the second-largest rainforest in the world. The Congo Basin spans across six countries—Cameroon, Central African Republic, Democratic Republic of the Congo, Republic of the Congo, Equatorial Guinea and Gabon.

Option b is incorrect: Nigeria is located in West Africa and is not part of the Congo Basin. It is situated to the west of Cameroon.

Option c is incorrect: South Sudan is located in East-Central Africa, and while it is geographically closer to the Congo Basin compared to Nigeria, it does not fall within the basin itself. The Congo Basin is mainly found to the south of South Sudan.

Option d is incorrect: Uganda is also located in East Africa, north of Rwanda and Tanzania. While it is relatively close to the eastern edge of the Congo Basin, it does not lie entirely within the basin.

Source: https://rainforests.mongabay.com/kids/slideshows/congo-rainforest-tour/congo-rainforest-tour-02.html

https://www.researchgate.net/figure/The-regional-study-area-encompasses-6-countries-in-the-Congo-Basin-A-national-scale_fig1_348163070