Creating separate Azure subscriptions for each department is the best solution for differentiated billing and administrative control. Each subscription can be associated with its own payment methods and access controls, keeping departmental resources and costs distinct and manageable. While management groups organize subscriptions, they do not directly allow for separate billing. Azure resource groups are a way to manage resources within a subscription but do not provide separate billing. Availability zones and regions are related to the physical location and availability of resources, not billing and administrative segmentation. Azure policies are a set of rules that help manage and enforce resource organization, but they do not separate billing either.