Data Guard Broker is a management tool that simplifies the configuration, management, and monitoring of Data Guard environments. The prerequisites for using Data Guard Broker include:
* The primary and standby databases must run the same version of the Oracle Database server (A):
This ensures compatibility between the primary and standby databases and enables seamless role transitions and data synchronization.
* Network connectivity to the primary database instance must be defined on the servers hosting the standby database instances (B): Proper network connectivity is essential for communication between the primary and standby databases, allowing for the replication of data and the synchronization of changes.
* If any database in the configuration is a RAC database, then the broker configuration files must reside in shared storage accessible by all database instances for all databases in the broker configuration (D): In Real Application Clusters (RAC) environments, shared storage ensures that all instances of the RAC database can access the broker configuration files, facilitating the management of the Data Guard environment across all instances.References:
* Oracle Data Guard Broker documentation
* Oracle Real Application Clusters Administration and Deployment Guide

Real-Time Query is a feature that allows queries to be run on a physical standby database while it is applying redo data. The relevant truths about it are:
* Setting standby_max_data_delay=0 requires synchronous redo transport (A): For the real-time apply feature to function with no data delay (zero delay), synchronous redo transport must be used. This setting ensures that the data on the standby database is as current as possible before queries are executed against it.
* Disabling Real-Time Query prevents the automatic start of redo apply when a physical standby database is opened read-only (C): If Real-Time Query is disabled, opening the standby database in read-only mode will not start the redo apply process automatically. Redo apply needs to be manually started to synchronize the standby database with the primary.References:
* Oracle Data Guard Concepts and Administration Guide

The DBMS_ROLLING package facilitates a rolling upgrade process across a Data Guard configuration. The START PLAN procedure in particular handles several critical actions, including:
Creating a guaranteed restore point on the standby databases (B): This ensures that the standby databases can be reverted to their state before the rolling upgrade process in case of any issues.
Building a LogMiner dictionary on the primary database instance (C): This is necessary for logical standby databases to interpret redo data during the SQL Apply process.
Creating a guaranteed restore point on the primary database (D): Similar to the standby databases, this ensures that the primary database can be reverted to a known good state if necessary.
Reference:
Oracle Database PL/SQL Packages and Types Reference
Oracle Data Guard Concepts and Administration Guide