Using Enhanced Availability, how often does the database on the standby CUA get updated?

Using Enhanced Availability, how often does the database on the standby CUA get updated?

A.
Every 5 minutes or 500 writes, whichever occurs first

B.
Active CUA sends all database updates immediately to the Standby CUA

C.
Whenever the active CUA writes a backup of its database to Centera

D.
User must define the frequency depending on the desired RPO