The suggested answer is A, E.
To ensure database availability for vCenter Server 6.x, two relevant options are available. Microsoft SQL Server 2012/2014 AlwaysOn Availability Groups (AG) provides high availability and disaster recovery solutions specifically designed for SQL Server databases, which are commonly used with vCenter Server. Microsoft Windows Server Failover Clustering offers a reliable method to ensure that the databases remain available in case of server failures, providing additional protection and ensuring minimal downtime. Other options like vCenter Server Watchdog and vCenter Server Heartbeat are focused on the availability of the vCenter Server itself, not specifically the database.