In Oracle, a sequence created with the GLOBAL keyword is available and can produce values across all sessions and instances. However, a sequence created with the SESSION keyword is only specific to the session it was created in. When the NEXTVAL is called for a sequence, it will increment according to the sequence's properties set during its creation.

Given the sequence creation statements and the actions performed:

The a sequence is global, which means it is available across the entire database, including the standby database with Real-Time Query enabled. So, when session 2 calls a.nextval, it will get the next value in the sequence, which is 21 since session 1 already retrieved 1.

The b sequence is session-specific, so when session 2 calls b.nextval, it will get the value 1 because for this new session on the standby, this is the first time the sequence is being accessed.

Therefore, the output for session 2 will be a output as 21 and b output as 1, which corresponds to Option C.

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.

The SQL apply lag on a logical standby database can be caused by several factors:

A: An undersized undo tablespace can lead to delays in SQL apply operations as it may not be able to handle the volume of undo records generated by the SQL apply process.

B: SQL apply operations that do full table scans can consume significant system resources, potentially leading to higher apply lag.

C: An increased number of bulk updates on the primary database may generate a large volume of redo data, which can cause apply lag if the logical standby cannot apply the changes quickly enough.

F: An undersized shared pool may affect the parsing and execution of SQL statements by SQL apply, which can contribute to the apply lag.

Option D is less likely to be a direct cause of SQL apply lag compared to bulk updates, as inserts generate new data rather than modifying existing data, which SQL apply can typically handle more efficiently.

Option E is incorrect because the size of the standby redo log files on the primary database impacts redo transport lag, not SQL apply lag.

To increase the number of applier processes on a logical standby database, the following changes can be made:

C: Increasing the value for the MAX_SERVERS SQL apply parameter would allow for more applier processes to be initiated, assuming that system resources permit.

D: Increasing the PARALLEL_MAX_SERVERS initialization parameter would allow for more parallel execution processes, which can be used by SQL apply to increase the number of applier processes.

Option A is incorrect as decreasing the number of archiver processes will not necessarily increase the number of applier processes; these are unrelated components.

Option B is incorrect because the 'FREPARER' processes do not exist, it seems to be a typographical error, and the 'REPARER' is not a valid Oracle process or parameter.

Option E is incorrect because the RECOVERY_PARALLELISM parameter controls the number of processes used for instance recovery and media recovery, not for SQL apply.

The rate of SQL apply on a logical standby database can be influenced by:

A: The number of PREPARER processes (which seems to be a typographical error and should read as PREPARER or similar) which prepare the redo data for the applier processes.

B: The number of coordinator processes on the standby database instance which coordinate the SQL apply activities.

C: The number of full table scans performed by SQL apply since full table scans can be resource-intensive and slow down the apply rate.

E: The number of applier processes which apply the redo data to the logical standby database.

Option D is incorrect as the size of the undo tablespace on the logical standby database is more likely to affect the SQL apply lag rather than the rate of SQL apply.

Option F is incorrect because the size of the shared pool would typically not influence the rate of SQL apply. The shared pool is more related to the caching of shared SQL and PL/SQL code and control structures.