The correct answer is B.
In order to get application launch working for resources managed by the Citrix Cloud Site, the Citrix Administrator needs to add the Citrix Cloud Connectors as Secure Ticket Authority (STA) servers on Citrix ADC and StoreFront. This will allow the Citrix ADC and StoreFront to communicate with the Citrix Cloud Site and authenticate user requests for application launch.
Decommissioning the old Citrix Delivery Controllers and Databases (option A) would not resolve the issue, as the problem is related to the communication between on-premises components and Citrix Cloud. Removing the Citrix policies on the old Citrix Delivery Controllers (option C) would also not solve the issue, as policies are not related to the communication between components.
Adding the Citrix Cloud Connectors as XML servers to the existing StoreFront servers (option D) is not the correct solution, as XML servers are responsible for enumerating and providing the list of available resources to the user, but they are not responsible for authenticating user requests for application launch.