Concept:A unitary system is adopted because centralised control helps unify diverse groups into one nation.
Explanation:In a unitary system, all major powers lie with the central government.
This creates uniform laws and policies across the whole country.
It reduces regional differences and prevents conflicting rules in different areas.
As a result, people from various regions feel part of one single political community.
Therefore, the main reason countries choose this system is to promote national integration and unity.
Answer:B. promote national integration