Concept:A federation is a system of government with two levels of rule, each deriving authority from a written constitution that is supreme.
Explanation:In a federation, the constitution is the highest law of the land.
Both the central government and the state governments must obey the constitution.
No government can override the constitutional provisions that define the powers of the other.
This makes the supremacy of the constitution an essential feature of a federation.
An unwritten constitution is not typical of a federation.
A unicameral legislature is not a required feature because federations often use bicameralism to represent states.
The right to secede is not allowed in a federation, as it would break the union.
Therefore, the correct feature from the alternatives is the supremacy of the constitution.
Answer:C. supremacy of the constitution