To reduce the redo apply lag on standby databases, one could increase the size of the standby redo log files. Larger redo log files can accommodate more redo data, which may reduce the frequency of log switches and allow for more continuous application of redo data. Additionally, lowering the DB_BLOCK_CHECKING parameter to MEDIUM or LOW on the standby databases can help improve redo apply performance. High block checking can impose additional CPU overhead during the application of redo data, potentially increasing apply lag times. By reducing the level of block checking, you can lessen this overhead and help reduce the apply lag .

* A fast recovery area must be configured (B): Flashback Database requires a fast recovery area to be set up because flashback logs are stored there. The fast recovery area is a unified storage location for all recovery-related files and activities.
* The database must be in ARCHIVELOG mode (C): Flashback Database operation relies on the ability to archive redo logs. Therefore, the database must be running in ARCHIVELOG mode for Flashback Database to be enabled.
References:
* Oracle Database Backup and Recovery User's Guide
* Oracle Data Guard Concepts and Administration Guide

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.

The ORA-03172: STANDBY_MAX_DATA_DELAY error indicates that the real-time query on the physical standby database is experiencing delays beyond the specified maximum data delay threshold. Increasing the network bandwidth (Option E) can enhance the speed at which redo data is transferred from the primary to the standby database, thereby reducing the likelihood of exceeding the STANDBY_MAX_DATA_DELAY threshold. Reducing I/O latency on the primary database's storage (Option B) ensures that redo data is generated and shipped more efficiently, further mitigating the risk of delay. These actions, focused on optimizing data transfer and processing speed, address the root causes of the ORA-03172 error in a synchronous Data Guard configuration operating in Maximum Availability mode.