Answer : 4. "Union of states"
Explanation :

(a) Federal: This term typically refers to a system of government where power is divided between a central authority and regional entities. India's political system is federal in nature as it includes both a central government and individual state governments.

(b) Federal with Strong Unitary Base: India's federal structure contains elements of both federalism and unitarism. While it is primarily federal, there are strong unitary features, meaning the central government has significant powers, especially in crucial areas like defense and national security.

(c) Confederation: This term refers to a loose union of states where they retain most of their independence and delegate only a few powers to a central body. India is not a confederation; it is a federal union where states have their own governments but are bound together under a strong central authority.

(d) Union of States: This accurately describes India's political structure as mentioned in Article 1 of the Indian Constitution. It signifies that India is a union comprising individual states and union territories, emphasizing both the unity of the country and the diversity and autonomy of its states.