What aspect of an organization's governance model should concern an IS auditor the most?

The most critical aspect of an organization's governance model from an IS auditor's perspective is the periodic review of the information security policy by management. This is important because the information security landscape is constantly evolving due to new threats, technology changes, and regulatory requirements.

Regular reviews ensure that the information security policy remains relevant and effective in addressing current risks and implementing best practices. It reflects proactive management oversight and commitment to maintaining a robust security posture. By continuously assessing and updating the policy, management can adapt to emerging threats, incorporate lessons learned, and align with organizational objectives. An effective governance model relies on the dynamic management of security policies to protect the organization’s assets and assure stakeholders of the organization’s integrity and reliability.

In contrast, while the other options are also relevant to governance and security, they do not carry the same weight of ongoing management involvement and adaptability that a periodic review entails. The existence of policies, for instance, is fundamental but doesn't ensure they are effective if they're not regularly evaluated and updated based on the latest information and trends.