Step 1: Understanding the Concept:
Judicial review is the power of the judiciary to examine the constitutionality of legislative enactments and executive orders.
Step 2: Detailed Explanation:
- Assertion (A): In cases like \textit{Kesavananda Bharati} and \textit{Minerva Mills}, the Supreme Court held that judicial review is an essential feature of the Constitution that cannot be taken away even by a constitutional amendment. Thus, it is part of the "Basic Structure".
- Reason (R): The primary purpose of judicial review is to maintain the supremacy of the Constitution, protect the fundamental rights of citizens, and maintain the check-and-balance system.
- Relationship: Judicial review is considered "Basic Structure" precisely \textit{because} it is the only mechanism that ensures the independence of the judiciary and prevents the legislature from encroaching upon the rights of the people. Without it, the Constitution would be at the mercy of the majority in Parliament.
Step 3: Final Answer:
Both A and R are correct, and R explains why judicial review is fundamentally indispensable to our constitutional scheme.