* The compatible parameter on a standby database that is applying redo, must be equal to or greater than the compatible parameter on the primary that is shipping redo to that standby (A):
This ensures that the standby database can apply redo from the primary, even after the primary has been upgraded. The COMPATIBLE parameter setting on the standby database should not preclude it from understanding the redo it receives.
* Modifications to the data dictionary on the primary database caused by the upgrade, are applied on a physical standby database (C): When the primary database undergoes a non-rolling upgrade, any resulting data dictionary changes are transmitted through redo data and applied to the physical standby database.
References:
* Oracle Database Upgrade Guide
* Oracle Data Guard Concepts and Administration Guide

In the context of Oracle Data Guard and container databases (CDBs) managed by Data Guard Broker:
* All broker actions execute at the root container for container databases (D): When using Data Guard Broker to manage a CDB, the actions performed by the broker are executed at the level of the root container. This is because the root container maintains the control and configuration information that applies to the entire CDB, including all of its pluggable databases (PDBs).
* After a role change, the broker opens all Pluggable databases (PDBs) on the new primary (E):
Following a role transition such as a switchover or a failover, Data Guard Broker ensures that all PDBs within the CDB of the new primary database are opened, which is essential to resume operations of the PDBs without manual intervention.References:
* Oracle Data Guard Broker documentation
* Oracle Multitenant Administrator's Guide

Transaction Guard provides benefits in terms of transaction consistency and recovery in a Data Guard environment:
* It provides application continuity by rolling back uncommitted transactions interrupted by a failover or switchover (B): Transaction Guard ensures that any uncommitted transactions at the time of an outage are rolled back consistently, thus preserving the integrity of the application's data and state.
* It protects against recoverable errors during a planned or an unplanned outage of a primary database (D): Transaction Guard offers protection against errors that can occur during outages, allowing applications to resume operations more quickly and reliably after recovery.References:
* Oracle Database High Availability Overview
* Oracle Real Application Clusters Administration and Deployment Guide

Snapshot standby databases are a feature of Oracle Data Guard that allows a physical standby database to be temporarily converted into a read-write database for testing or other purposes. The true statements about snapshot standby databases are:
FLASHBACK DATABASE is enabled automatically on a snapshot standby database after converting it from a physical standby database if not already enabled (B): When a physical standby is converted to a snapshot standby, FLASHBACK DATABASE is automatically enabled to allow the database to be easily reverted back to its original state.
FLASHBACK DATABASE is enabled automatically on a physical standby database as part of the conversion into a snapshot standby database, if not already enabled (C): As part of the conversion process, FLASHBACK DATABASE is turned on to ensure that changes made while the database is in snapshot standby mode can be undone.
A snapshot standby database may be opened read-write (E): Once a physical standby is converted to a snapshot standby, it can be opened for read-write operations, allowing for testing and other tasks that require a writable database.
Reference:
Oracle Data Guard Concepts and Administration
Oracle Database Backup and Recovery User's Guide