In a Data Guard environment, you can offload the backups of certain database components to a physical standby database. Incremental backups of a standby database are interchangeable with the primary database, meaning you can apply a backup taken on a standby database to a primary database and vice versa. This includes control files, data files, and archive logs. Backups of control files and nonstandby control files are interchangeable. You can restore a standby control file on a primary database and a primary control file on a physical standby database, demonstrating their interchangeability and the ability to offload control file backups to one database in a Data Guard environment.

Global Sequences are Oracle sequences that generate unique values across multiple instances in an Oracle RAC or a Data Guard configuration. Regarding their behavior and performance when connected to a physical standby database with Real-Time Query enabled:
A: The usage of Global Sequences can indeed have a performance impact on the primary database due to the need to generate unique values that are consistent across both primary and standby databases.
D: The performance impact on the physical standby database may occur if the CACHE size is too small. This is because the standby database will frequently have to access the primary database to replenish the cache, which can increase the load and potentially lead to performance degradation.
E: Global Sequences should have the NOORDER and CACHE options set. The NOORDER option ensures that sequence numbers are provided without guaranteeing sequence order, thus improving scalability and performance. The CACHE option is used to specify how many sequence values will be held in memory for faster access.
Option B is incorrect as the LOG_ARCHIVE_DEST_n parameter's definition for standbys pointing back to the primary does not directly pertain to the creation of sequences.
Option C is incorrect because there is no requirement that the size of the cache for a sequence must be at least 100. The CACHE size can be set to a different number based on specific use cases or performance considerations.

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

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.
Reference:
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.