To ensure that all servers for the accounting department are backed up and any new servers added to the department are automatically included in the protection group, the best approach is to use an Active Directory (AD) based protection group. Therefore, the correct answer is A:
Create an Active Directory based protection group targeted at an Active Directory security group.
By leveraging an AD-based protection group, the Veeam administrator can specify an AD security group that encompasses all user accounts or computer accounts (in this case, servers) associated with the accounting department. As new servers are introduced and added to this AD security group, Veeam Backup & Replication will automatically detect these new members during its periodic rescan (which can be scheduled as per the administrator's preferences) and include them in the protection group. This method not only automates the inclusion of new servers into the backup routine but also simplifies the management of backup policies for dynamically changing environments such as the accounting department's server infrastructure.