Which of the following supplemental security devices implements the DLP (data loss prevention) security control?

The correct choice is the XML gateway, as it plays a crucial role in implementing Data Loss Prevention (DLP) security controls. XML gateways are specifically designed to monitor and control XML traffic between a web application and external entities, acting as a filter that can enforce security policies. This includes the ability to inspect, validate, and transform XML data in real-time, ensuring that sensitive information is not inadvertently shared or exposed.

DLP focuses on identifying, monitoring, and protecting sensitive data from unauthorized access or sharing. XML gateways facilitate this by providing mechanisms such as data inspection and filtering, which are essential for DLP strategies. They can prevent the transmission of confidential information and enforce compliance with organizational policies regarding data handling.

In contrast, the other options—API gateways, web application firewalls, and database activity monitoring—focus on distinct aspects of security management. API gateways primarily manage and expose APIs securely, web application firewalls defend against external threats like SQL injection and cross-site scripting, while database activity monitoring centers on ensuring the integrity and availability of database environments. While these might contribute to an overall security strategy, they do not specifically focus on DLP as XML gateways do.