Based on Oracle Database 19c: Data Guard Administration documents, the three measures that can prevent clients from connecting to the wrong database instances during switchovers, failovers, or regular operations in a Data Guard environment are:
* B. The standby database services must be defined statically with the Listeners running on the standby database hosts.
* D. The client applications must use the correct TNS entries when requesting connections to the database instances.
* E. Client TNS entries for the databases use the correct service names for the intended service.
* In an Oracle Data Guard configuration, correctly configuring Oracle Net Services (including TNS entries and listeners) is crucial for ensuring that clients connect to the appropriate database instance, whether it's the primary or standby. Defining services on the standby database and associating them with listeners ensures that client applications can connect to the standby when needed, especially useful in a role transition or when the standby is open for read-only access or real-time query. It's essential that TNS entries used by client applications specify the correct service names that correspond to the intended
* database roles, such as primary or standby. This setup facilitates seamless connectivity to the appropriate instance based on the role, especially critical during switchovers and failovers when the roles of the databases change.
* References:Oracle's Data Guard concepts and administration guide provides extensive information on configuring network services for Data Guard environments, ensuring that applications connect to the correct database instance based on the current role of the databases in the Data Guard configuration.

When SYNC redo transport mode is combined with Maximum Protection mode, it ensures that no data loss will occur (requirement 1). The physical standby can be converted to a snapshot standby (requirement 2) and later to a logical standby database (requirement 5), satisfying both transformation requirements. Transactions commit as soon as redo data is received by the standby database (requirement 3). The availability of the primary is not dependent on the standby database in Maximum Protection mode, as the primary database will halt if the standby cannot acknowledge the redo (requirement 4), thus indirectly ensuring its availability. It is also possible to deploy Real Application Clusters on both the primary (requirement 6) and the physical standby database (requirement 7), providing high availability and scalability.
Reference
Oracle Data Guard documentation detailing the requirements for different database roles, protection modes, and redo transport modes, as well as the capabilities and limitations of each configuration.

The process of performing a rolling release upgrade in a Data Guard environment using a logical standby database generally involves these steps:
* Raise the compatibility level on both databases (1): Ensuring both the primary and logical standby databases are operating with the same and correct compatibility level is essential before starting the upgrade process.
* Upgrade the logical standby database (5): Apply the database upgrade to the logical standby first, which allows the primary database to continue serving the workload without interruption.
* Restart SQL Apply on the upgraded logical standby database (2): Once the logical standby has been upgraded, SQL Apply must be restarted to apply the redo data from the primary database, which is still running the earlier version.
* Perform a switchover to the logical standby database (4): After confirming that the logical standby database is successfully applying redo data, perform a switchover to make it the new primary database.
* Upgrade the old primary database (6): With the new primary database now in place, upgrade the old primary database (which is now the new standby) to the new Oracle Database release.
* Start SQL Apply on the old primary database (3): Finally, start SQL Apply on what is now the standby database to synchronize it with the new primary database.References:
* Oracle Data Guard Concepts and Administration Guide
* Oracle Database Upgrade Guide