Final answer:
The term for when a user's session validation item is stolen for unauthorized access is known as C. Session hijacking. This breach allows an attacker to impersonate a user, typically by capturing session cookies on unsecured networks.
Step-by-step explanation:
The term that describes when the item used to validate a user's session, such as a cookie, is stolen and used by another to establish a session with a host that thinks it is still communicating with the first party is C. Session hijacking. This type of security breach occurs when a third party gains unauthorized access to a user's session token, allowing them to impersonate the user and potentially access sensitive information or perform actions on behalf of the legitimate user. Session hijacking can be executed through different methods, including sidejacking, where attackers eavesdrop on a network and capture session cookies over unsecured connections like public Wi-Fi networks.