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.
Reference:
Oracle Database Backup and Recovery User's Guide
Oracle Data Guard Concepts and Administration Guide

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.
Reference:
Oracle Database Upgrade Guide
Oracle Data Guard Concepts and Administration Guide

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.
References: Oracle's documentation on sequences and their behavior in a Data Guard environment provides insights into the performance considerations and best practices for using sequences, particularly in a Real-Time Query context.