To restore a Cloud SQL for PostgreSQL database to the last known pristine version in a high availability configuration, the most effective method is to use the Cloud SQL point-in-time recovery (PITR) feature. This allows you to restore the database to a specific point in time, which is crucial for transactional applications where data integrity and minimal data loss are priorities. Creating a clone from a read replica (options A and B) might not guarantee the pristine state of the database as the replica could also be affected by the issues. Importing a dump file from Cloud Storage (option D) is not ideal because it might not reflect the most recent state of the database before the issues occurred. Therefore, option C is the correct answer as it directly addresses the need to restore to a specific, known good state with minimal data loss.