Explanation:

- Dual Government: India has two levels of government, central and state, creating a federal structure. Each has its respective powers, which is a hallmark of federalism.

- Division of Powers: The Constitution divides powers between the central government and the states, which is essential for a federal system.

- All India Services: These services operate across both the central and state levels and undermine the autonomy of states, leaning more towards a unitary feature.

- Written Constitution: The Indian Constitution is a single, comprehensive document that outlines the framework of governance and is necessary for defining federal principles.

Correct Answer: Option 3 - All India Services