Option A is correct. For traffic to be sent through a GRE tunnel, there must be a route that directs the traffic into the tunnel. This is typically accomplished through the use of a static route or a dynamic routing protocol.
Option B is correct. All intermediary devices must have a route to the tunnel endpoints. In real- world scenarios, the tunnel endpoints for a tunnel going over the Internet must have globally reachable internet addresses. Otherwise, intermediate routers in the Internet cannot forward the tunneled packets.

To isolate management traffic in a non-default routing-instance on Junos-based devices, you can use the set system management-instance and set routing-instances mgmt_junos commands. set system management-instance: This command associates the management interface (usually named fxp0 or em0 for Junos OS, or re0:mgmt-* or re1:mgmt-* for Junos OS Evolved) with the non- default virtual routing and forwarding (VRF) instance. After you configure the non-default management VRF instance, management traffic no longer has to share a routing table with other control traffic or protocol traffic.
set routing-instances mgmt_junos: This command creates a new routing instance named mgmt_junos. The name of the dedicated management VRF instance is reserved and hardcoded as mgmt_junos; you cannot configure any other routing instance by the name mgmt_junos.
Therefore, options C and D are correct. Options A and B are not correct because they attempt to assign an interface to the mgmt_junos routing instance, which is not necessary for isolating management traffic.