What type of exercise would best validate that an organization's cybersecurity measures are effective against known threat actor groups?

The best choice for validating an organization's cybersecurity measures against known threat actor groups is simulation exercises. These exercises are designed to create a realistic environment where teams can practice responding to specific threat scenarios that mimic the tactics, techniques, and procedures used by actual threat actors.

Simulation exercises allow organizations to evaluate their detection and response capabilities in a controlled but realistic setting. This can include everything from identifying breaches to mitigating attacks based on real-world tactics used by known adversaries. Additionally, simulations often involve various departments, pushing teams to work together and ensuring that communication channels are effective during a cybersecurity incident.

The other options serve important functions within a cybersecurity framework but do not focus solely on the validation against known threats as effectively. Penetration testing primarily assesses vulnerabilities and weaknesses in systems without necessarily mimicking specific threat actor behaviors. Red teaming also assesses the organization's defenses but is typically more focused on the broader aspect of security posture rather than validating against known attackers. Tabletop exercises are strategic discussions that help in planning responses but lack the dynamic and real-world applicability of simulations.