Comprehensive and Detailed Explanation:

For highly sensitive user data, you need to isolate applications while minimizing management overhead. The best practice is:

One GKE cluster per environment (Development and Production) → This provides a clear separation of concerns and avoids security risks from running different environments in the same cluster.

Each application in its own namespace → Namespaces provide logical isolation for different applications within the same cluster, reducing unauthorized access risks.

????Why not other options?

A (Single cluster for org with multiple namespaces for apps & envs)❌→ Bad security practice because mixing production and development in the same cluster increases the risk of privilege escalation.

C (Single cluster for org with namespaces per app)❌→ Still mixes development and production in the same cluster, violating isolation requirements for sensitive data.

D (One cluster per app)❌→ High operational overhead; unnecessary complexity for small to medium-scale deployments.

????Official Reference:

GKE Multi-Tenancy Best Practices

GKE Security Hardening