Which option requires a separate subscription on top of a firewall?

The correct choice highlights a feature that operates distinctly from the base firewall functionality and thus necessitates an additional subscription. PAN-DB URL filtering is a service that enhances security by allowing or denying access to URLs based on their categorization. This service involves a subscription because it relies on a continuously updated database that categorizes websites, enabling real-time analysis and threat intelligence.

When a user intends to leverage URL filtering capabilities beyond the inherent features of the firewall, they must subscribe to the PAN-DB service to access this categorized URL database. The integration of such service also ensures the firewall can proactively respond to web traffic based on the latest threat landscape.

The other options, while they may enhance the capabilities of a firewall, typically fall under the standard functionalities or can be managed through built-in features without requiring a separate license. For instance, data filtering, file blocking, and DoS protection usually utilize the existing resources of the firewall and are often included in the base product's features, thus not necessitating an extra subscription specifically for their utilization.