In an Oracle Active Data Guard environment:
* B: Read-mostly reporting applications that utilize global temporary tables to store session-specific data can be effectively offloaded to an Active Data Guard standby database, reducing the load on the primary database.
* C: Sequences can be used with global temporary tables on an Active Data Guard standby database to support certain types of read-mostly applications, though some restrictions on sequence use may apply.
* E: In Oracle Database 19c and later, DML redirection allows DML operations performed on an Active Data Guard standby database to be transparently redirected to the primary database. This is part of the DML Redirection feature.
Option A is incorrect because not all PL/SQL blocks run on an Active Data Guard standby database can be redirected to the primary database. Some PL/SQL executions, specifically those that would attempt to make changes to the database, are not supported on the standby.
Option D is incorrect because DDL operations on private temporary tables are not redirected; instead, private temporary tables are session-specific and are not persisted on disk, so they do not generate redo and are not applicable to an Active Data Guard standby.
References: The behavior and capabilities of Active Data Guard standby databases are documented in the Oracle Data Guard Concepts and Administration guide and the Active Data Guard documentation.

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.

For seamless client redirection in a Data Guard environment, the following steps should be taken:
* Create a database service on the primary database that is started automatically by a trigger when the database role is PRIMARY (B): This ensures that the service is only available on the primary database and is automatically started after a role transition due to switchover or failover.
* Modify the connection descriptors used by client applications to include all the standby hosts and connect to the database instance using that service name (B): Client applications use the connection descriptors that include all potential primary hosts (i.e., the current primary and all standbys). This enables clients to connect to whichever database is currently acting as the primary using the service name.References:
* Oracle Data Guard Concepts and Administration Guide
* Oracle Real Application Clusters Administration and Deployment Guide