You determine that business units have Azure resources spread across different Azure resource groups.
You need to make sure that resources are assigned to their proper cost centers.
What should you do?
You should create taxonomic tags and assign them at the resource level. Tags in Azure are key-value string pairs that administrators can associate with Azure resources for logical organization. Identifying cost centers is an excellent use case for tags. Because corporate divisions own Azure resources spread across different Azure resource groups, you have to assign cost center tags at the resource level. Wherever possible, it is best practice to organize related resources into the same resource groups because you can then bulk-assign taxonomic tags to all contained resources in a single operation.
You should not create taxonomic tags and assign them at the resource group level. The scenario states that business units have resources spread across different resource groups. If you assign a particular cost center tag at the resource group level, then you likely will mis-tag contained resources owned by another business unit.
You should not deploy the Enforce tag and its value Azure Policy. Doing so enforces the presence of a single specified tag and value pair. In this case, the scenario states that the organization has more than one cost center and therefore needs more than one taxonomic tag.
You should not deploy the Enforce tag and its value on resource groups Azure Policy for the same reasons. The company has more than one cost center, and the business units have resources spread across multiple resource groups.