Which tool should an administrator use to find all resources across subscriptions that are missing a cost center tag?

Azure Resource Graph is the correct tool because it allows you to explore and query your Azure resources at scale across multiple subscriptions. You can use Kusto Query Language (KQL) to filter for resources that do not have a specific tag, such as a 'cost center' tag. Azure Blueprints are used for deploying standardized environments and compliance, not for querying existing resources. Azure Sentinel is a Security Information and Event Management (SIEM) solution for security analytics. Azure App Insights is an Application Performance Management (APM) service for monitoring application performance and usage.