To maximize uptime for your application while meeting the RTO and RPO within 30 minutes, the best approach is to deploy Cloud SQL for PostgreSQL in a regional configuration with HA enabled and create a cross-region read replica. This setup ensures high availability within the primary region and provides a quick disaster recovery option by promoting the read replica in another region as the primary node in case of a failure. Option A provides high availability but lacks a quick disaster recovery mechanism. Option B relies on backups for disaster recovery, which may not meet the RPO and RTO requirements as effectively as promoting a read replica. Option D suggests migrating to Cloud Spanner, which, while offering high availability and global scalability, requires significant changes to the schema and application, making it less ideal for a straightforward migration aiming to maximize uptime with minimal changes.