Comprehensive and Detailed Explanation: The most effective way to prevent unauthorized IoT devices from connecting is through network access control (NAC), which enforces authentication and authorization before allowing a device onto the network. * Vulnerability scans (A): Identify weaknesses but do not actively prevent device connections. * Reviewing IoT configurations (B): Focuses on existing devices, not unauthorized ones. * Policies (D): Provide guidance but do not enforce technical prevention. # ISACA Reference: CISA Review Manual 27th Edition, Domain 5, section on network security and endpoint access control.