When a site-to-site VPN is used, which protocol is responsible for the transport of user...

IPsec. Security fundamentals require distinguishing identity, authorization, accounting, encryption, inspection, and physical protection. Cisco CCNA 200-301 v1.1 includes this under Security Fundamentals, where the exam expects engineers to recognize the device behavior that actually produces the required outcome. The question is best solved by reading the operational clue rather than choosing a familiar acronym. The incorrect choices apply a security term in the wrong control category or protect a different part of the system. In a production network, the wrong choice would normally create an outage, leave a management or security gap, or send troubleshooting toward the wrong subsystem. The selected answer is the one that matches the control-plane, data-plane, wireless, security, services, or automation mechanism described in the question. That is why it remains the verified answer for this item.