Which testing methodology runs against systems that can tune their focus of security?

The correct choice, which is RASP (Runtime Application Self-Protection), is designed to operate within the application in real-time during its execution. This methodology allows it to monitor the application's behavior and security posture continuously, enabling it to respond to threats dynamically as they occur. RASP integrates into the application environment and can adjust its security focus based on real-time interactions and conditions it detects.

This adaptability is a critical feature since it means that RASP can fine-tune its defensive mechanisms based on immediate context and usage patterns, rather than relying solely on pre-defined rules or external testing frameworks. By doing so, it enhances the application's resilience against emerging attacks, providing a more nuanced and responsive security approach.

In contrast, the other options, such as DAST (Dynamic Application Security Testing) and SAST (Static Application Security Testing), typically analyze applications either during their execution in a more external manner or while they are not running, respectively. REST (Representational State Transfer) is not a testing methodology; it's an architectural style for designing networked applications. Therefore, RASP stands out as the methodology that inherently possesses the capability to adjust its security measures actively, aligning it with the requirement of tuning its focus on security.