The correct answer is C. Determine the level of employee adherence to the policies.
To obtain assurance that IT policies have been effectively implemented, the auditor must determine whether employees actually follow the policies in daily operations. A policy may be well written, approved, and aligned with standards, but it is not effectively implemented unless users, administrators, managers, and process owners comply with it.
ISACA guidance on policy compliance explains that assurance over compliance requires collecting sufficient information through methods such as direct observation, metrics, corroboration, tests, controls, audits, self- assessments, and critical questioning. ISACA's CISA Exam Content Outline also includes evaluating the organization's management of IT policies and practices, including compliance with legal and regulatory requirements.
Option A is not the best answer because external certifications may provide some comfort, but they do not prove that the organization's own employees are consistently following internal IT policies.
Option B is not the best answer because benchmarking shows whether policies are well designed compared with industry standards. It does not prove that the policies are implemented or followed.
Option D is not the best answer because security breaches are lagging indicators. A lack of breaches does not prove policy effectiveness, and an increase in breaches may be caused by many factors unrelated to policy implementation.
This question maps mainly to Governance and Management of IT because IT policies, standards, procedures, practices, and compliance with those practices are part of IT governance.
References: ISACA CISA Exam Content Outline, Domain 2; ISACA article on determining policy compliance.