Comprehensive and Detailed Step-by-Step Explanation: To ensure that aservice vendor maintains required control levels, direct verification throughonsite assessmentsis the most effective approach. * Option A (Correct):Onsite assessmentsallow auditors todirectly reviewcontrols, procedures, and evidence of compliancein real time, ensuring that service levels are being met. * Option B (Incorrect):Unannounced vulnerability assessments may violatecontractual agreementsand ethical considerations. * Option C (Incorrect):Reviewing theSLAensures agreement terms are clear but doesnot verify actual compliance. * Option D (Incorrect):AControl Self-Assessment (CSA)is useful but relies onvendor-provided information, which may be biased or incomplete. Reference:ISACA CISA Review Manual -Domain 4: Information Systems Operations and Business Resilience- Covers third-party risk management and audit approaches.