Concept:
Article 1 of the Indian Constitution describes India as a "Union of States." This phrasing was deliberate and reflects the unique nature of Indian federalism.
Step 1: Evaluating Assertion (A).
Assertion (A) is correct. The term "Union" was used by Dr. B.R. Ambedkar to emphasize two things: first, that the Indian federation was not the result of an agreement by the states (like the USA); and second, that the states have no right to secede from the federation.
Step 2: Evaluating Reason (R).
Reason (R) is correct. India is often described as "Quasi-federal" or a "Federal structure with a unitary bias." It has federal features (dual government, division of powers, written constitution) but also strong unitary features (All India Services, Governor's appointment by the Centre, Emergency provisions).
Step 3: Analyzing the Explanation.
Reason (R) correctly explains Assertion (A). The reason India is described as a "Union" (rather than a loose Federation) is precisely because of these unitary features that ensure national integrity and central authority over the constituent units. The unitary features provide the "glue" that makes the federal structure a "Union."