(C) Federal
Belgium solved its problem of majoritarianism by strengthening the Federal form of government, giving constitutional powers to regional governments independent of the central government.
Source: What is federalism?, Chapter 2
---
The passage explicitly states: "Belgium shifted from a unitary to a federal form of government" after the 1993 constitutional changes. Regional governments were given constitutionally guaranteed powers not dependent on the central government. This is the key fact examiners expect.