Hint: A federal system is one where the political authority is distributed between two tiers of Government, one at the central level and one at the state level. The federal structure allows in holding together various ethnic sections of the population and accommodates ethnic diversity. Federalism is a feature of democracy, South Africa, Namibia, Tunisia have a better democracy score than others.
Congo: Congo has a semi-presidential republic form of Government. It has an executive, judiciary, and a legislative branch. There is no federal structure, but a form of decentralization exists. The majority of the powers are vested in the Central government.
Sudan: Sudan has a Presidential representative democracy form of Government. The President is the head of the state, head of government, and head of the military. Despite having some form of federal structure, it remains irrelevant since the country is totalitarian, and decisions are unilateral by the head of the state.
South Africa: South Africa has a federal system of Government. It is a constitutional democracy where powers are divided between three tiers of government- The Central, the state, and the local level of Government. They possess legislative and executive authorities in their own domains. The Constitution declares these tired as independent and interrelated.
Tanzania: Tanzania has a unitary presidential democratic republic form of government. The President exercises both the position of the head of the state and head of the government. Tanzania is not a formal federal nation.
So, the correct answer is Option C.
Note: A federal system can be said as a formal compact between different levels of government. Some forms of decentralization might lie in every country, but in order to be called a complete federal nation, one must fulfill all the criteria, and also, successfully implement them. Civil wars in Africa have undermined the working of an effective Government.