In Microsoft Azure, Network Security Groups (NSGs) are used to filter network traffic to and from Azure resources in an Azure Virtual Network (VNet). NSGs can be associated with subnets or network interfaces (NICs) but are scoped by region - meaning that an NSG can only be applied to resources within the same Azure region.
From the scenario:
* NSG1 is located in the East US region.
* VNet1 and VNet2 are located in the West US region.
* VNet3 is located in the East US region.
Therefore, based on Microsoft Azure's network management rules, NSG1 can only be applied to subnets or network interfaces in VNets located in the same region (East US). This means NSG1 can only be assigned to VNet3, since both reside in the East US region.
As stated in Microsoft Learn (Azure Networking documentation):
"A network security group (NSG) must exist in the same region as the virtual network or subnet to which it is associated. You cannot link or associate an NSG across regions." This regional boundary ensures proper security control enforcement and routing consistency within Azure's networking infrastructure.