Third is NO. You can configure UDRs between two subnets in different virtual networks.

3 - keyword is only - so its No A User Defined Route (UDR) in Azure can control network traffic not only within a single Virtual Network (VNet) but also between different subnets, VNets (through VNet peering), and even to on-premises networks if connected via VPN or ExpressRoute. UDRs provide a way to define how traffic is routed in the network, offering flexibility beyond the default system routes Azure provides.

Think should be Y, Y, Y

Is Y,Y,N The statement "A user-defined route (UDR) can only control network traffic between subnets of a single virtual network" is incorrect. In Azure, user-defined routes (UDRs) can indeed control network traffic within a single virtual network, but they can also influence traffic to destinations outside of the virtual network. This includes routing traffic to on-premises networks through VPN gateways, peered virtual networks, or even the internet via a Network Virtual Appliance (NVA).

Is Y,Y,N