To which subnets can an Azure network security group (NSG) be applied if it is not in the same resource group as the virtual networks?

An Azure Network Security Group (NSG) can be applied to subnets within different virtual networks across various resource groups, as long as those virtual networks are in the same region. This means that the NSG can regulate traffic to any associated network interfaces or subnets in different virtual networks, which is a significant aspect of Azure’s flexibility in managing network security.

The concept of resource groups is primarily for organizing resources; it doesn't impose strict limitations on the ability to connect and manage networking elements across different resource groups. Therefore, when an NSG is created, it can effectively control the flow of inbound and outbound traffic for each of the subnets associated with every virtual network that it is linked to, regardless of which resource group they belong to. That’s why the correct response encompasses all the subnets from the specified virtual networks, as the capacity to apply the NSG is not limited to a specific resource group boundary.