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 .

The DBMS_COMPARISON package, used for comparing and converging data objects within a single database or between databases, requires that the databases involved in the block comparison be at least mounted (A). This allows the procedure to access the data blocks for comparison. Additionally, the progress of long-running operations such as block comparison can be monitored using the dynamic performance view V$SESSION_LONGOPS (D), which provides information on the operation's progress and estimated completion time.
References:Oracle Database PL/SQL Packages and Types Reference provides comprehensive details on the DBMS_COMPARISON package, including its procedures and how to monitor their progress. Additionally, Oracle Database Reference explains the V$SESSION_LONGOPS view, which is commonly used for monitoring long operations in the database.

Creating dedicated database services for each database instance (Option D) and utilizing event triggers to manage these services based on the role of the database (Option E) ensure that clients connect to the appropriate database instance based on its current role and state. This approach leverages the flexibility and control provided by Oracle Net services and database event management to direct client connections to the suitable primary or standby instance, enhancing the overall robustness and reliability of the Data Guard environment. Reference: Based on Oracle Database 19c best practices for managing connectivity and services in a Data Guard setup, including the use of role-based services and event-driven service management.