A query may not be offloaded to the HeatWave cluster for execution due to the following reasons:
A:Theuse_secondary_enginesession variable is OFF: This setting controls whether queries should be offloaded to the secondary engine (HeatWave). B.The client is connected to the DB system instead of the HeatWave cluster: For queries to be offloaded, the client must be connected to the HeatWave cluster. E.A required column is excluded from the RAPID secondary engine: If a necessary column is not available in the HeatWave (RAPID) secondary engine, the query cannot be offloaded.

For a MySQL DB system with five active read replicas and a workload consisting mostly of reads, the fastest query response time is achieved by:
C:Connect to the read replica load balancer to perform read-only operations: The read replica load balancer distributes the read workload across all replicas, optimizing performance and response time.

When automatic backup retention of a DB system is disabled,deleting the DB system(Answer C) will delete all automatic backups of the DB system.
* Automatic Backups: These are backups created automatically by the OCI service to ensure data protection.
* Backup Deletion: If automatic backups are disabled and you delete the DB system, all associated automatic backups are also deleted. This operation is irreversible, and you should ensure that necessary backups are taken before deleting the DB system.
References:
* OCI MySQL Database Service Backup and Restore

Data loaded into a MySQL HeatWave cluster:
B:Data is partitioned across cluster nodes for scalability: This allows the system to distribute the workload and handle large datasets efficiently. D.Data is stored in hybrid columnarformat to speed up analytic queries: The hybrid columnar format optimizes storage and speeds up query performance, especially for analytic workloads.

The MySQL HeatWave AutoML routine that evaluates a model isML_SCORE. This routine scores a trained machine learning model on a new dataset and evaluates its performance.