You must enable communication between VM1 and VM4 while minimizing administrative effort. What action should you take?

Establishing peering between VNET1 and VNET3 is the correct action. This allows virtual networks to communicate directly using private IP addresses, acting as if they are part of the same network. This is the most straightforward and administratively efficient way to enable communication between VMs in different virtual networks. Creating an NSG and associating it with both VM1 and VM4 would control traffic flow but wouldn't enable communication between separate virtual networks. NSGs filter traffic within a subnet or on a NIC, not between distinct VNETs. Assigning VM4 a specific IP address (10.0.1.5/24) is a configuration detail for the VM itself and doesn't resolve the cross-VNET communication issue. Creating a user-defined route from VNET1 to VNET3 is a more complex solution that would require additional configuration and management compared to the simplicity of VNet peering.