How does Amazon RDS multi Availability Zone model work?

A. A second, standby database is deployed and maintained in a different availability zone from master, using synchronous replication.
B. A second, standby database is deployed and maintained in a different availability zone from master using asynchronous replication.
C. A second, standby database is deployed and maintained in a different region from master using asynchronous replication.
D. A second, standby database is deployed and maintained in a different region from master using synchronous replication.