Explanation:
Box 1: RG1, RG2, or RG3
The resource group stores metadata about the resources. When you specify a location for the resource group, you're specifying where that metadata is stored. The location of the RG doesn't influence the choice of the location of VM. best practice would be to create the VM1 in the RG1 because the scale set is in RG1. And Microsoft recommends that resources contained in a Resource Group share the same resource lifecycle.
Box 2: West US only
You can add the virtual machine to a scale set in the same region, zone, and resource group.
Reference:
https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/overview
https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets- orchestration-modes