Correct Answer: A, C, DWhen facing registration issues with Virtual Delivery Agents (VDAs) in a Citrix Virtual Apps and Desktops environment, the engineer should check the following three items: time synchronization (NTP) configuration of the VDA machines with the domain, the VDA machines' registry setting for ListOfDDCs entries, and DNS configurations on either the VDA machines or the Delivery Controller servers. Time synchronization is crucial for proper functioning and security. The ListOfDDCs in the registry ensures that VA machines have accurate information to locate the Delivery Controllers. DNS configurations are essential for network communication and resolving hostnames. Other options like SSL certificates and firewall settings are generally not required for registration issues.