What should be documented in the event of an incident involving classified information?

The requirement to document a complete account of the incident involving classified information, including actions taken, is crucial for several reasons. First, thorough documentation ensures that all details are captured accurately, which is essential for understanding the nature and impact of the incident. This comprehensive account aids in conducting an effective investigation, assessing the breach's severity, and determining whether any policies or procedures were followed appropriately.

Additionally, having a detailed record serves as a basis for any necessary remediation actions and helps in improving future security measures. It enables an organization to learn from the incident, preventing similar occurrences in the future. Documenting actions taken is particularly important for ensuring accountability and complying with regulatory requirements, as well as for potential reporting obligations to authorities.

In contrast, limiting documentation to just the date and time, individuals involved, or financial costs would not provide the necessary context or detail needed to fully understand the incident's implications or to act on it effectively. Thus, the thorough documentation as indicated in the correct answer is essential for effective incident management and future prevention.