Given the resources shown in the table, to which subnets can NSG1 be applied?

Network Security Groups (NSGs) can only be associated with subnets within the same virtual network (VNet) as the NSG itself. In this scenario, NSG1 is located in VNet3. Therefore, NSG1 can only be applied to subnets that are also part of VNet3. It cannot be applied to subnets in VNet1 or VNet2 because they are distinct virtual networks. This is a fundamental security boundary in Azure networking, ensuring that network security rules are enforced within their defined VNet scope.