Initialization variables(Answer A) cannot be modified in MySQL custom configurations. These variables are set at the start of the MySQL service and are not changeable without restarting the service.
* Initialization variables are parameters that define how the MySQL server initializes.
* Changes to these variables require restarting the MySQL server to take effect.
* Examples include variables likeinnodb_buffer_pool_size, which determine memory allocation settings on startup.
References:
* MySQL Configuration Variables Documentation

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.

To enable high availability for a stand-alone DB system, the following conditions are required:
* The DB system must be active(Answer D): The DB system needs to be in an active state to enable high availability features.
* The DB system automatic backup must be enabled(Answer E): Automatic backups must be enabled to ensure data protection and facilitate failover operations.
Steps:
* Ensure the DB system is running and operational.
* Enable automatic backups through the OCI Console or API.
References:
* MySQL High Availability Documentation
* Automatic Backups Documentation

Creating a new DB system with data import provides the benefit of faster import with minimal logging. This process is optimized to import large datasets efficiently and quickly, reducing the amount of logging overhead and ensuring a rapid setup of the new DB system.

In a high-availability DB system, the placement of primary and secondary instances should ensure redundancy across different availability domains (ADs) and fault domains (FDs) to minimize downtime and data loss. The placement mentioned in option D is not valid because itplaces the primary and secondary instances in the same availability domain and fault domain, which does not provide high availability.