Authentication is a control that verifies the identity of a user or a system that tries to access a computer system or network. Authentication can be based on something the user or system knows (such as a password or a PIN), something the user or system has (such as a token or asmart card), or something the user or system is (such as a fingerprint or a retina scan). Authentication is a crucial control for preventing unauthorized or malicious access to a system or network, as well as for ensuring the accountability and traceability of the actions performed by the user or system. If the authentication control is compromised, it means that the user or system can bypass or break the verification process and gain access to the system or network without being identified or authorized. This can expose the system or network to various threats, such as data theft, data corruption, data leakage, or denial of service. Therefore, the authentication control has most likely been compromised if a system administrator identifies unusual activity indicating an intruder within a firewall. A firewall is a device or a software that monitors and filters the incoming and outgoing network traffic based on predefined rules and policies. A firewall can help to protect the system or network from external or internal attacks by blocking or allowing the traffic based on the source, destination, protocol, or content. However, a firewall cannot prevent an intruder from accessing the system or network if the intruder has already authenticated or impersonated a legitimate user or system. The other options are not the most likely controls to be compromised if a system administrator identifies unusual activity indicating an intruder within a firewall, although they may be affected or related. Data validation is a control that checks the accuracy, completeness, and quality of the data that is entered, processed,or stored by a system or anetwork. Data validation can help to prevent or detect data errors, anomalies, or inconsistencies that may affect the performance, functionality, or reliability of the system or network. However, data validation does not prevent or detect unauthorized or malicious access to the system or network, as it only focuses on the data, not the user or system. Identification is a control that assigns a unique identifier to a user or a system that tries to access a computer system or network. Identification can be based on a username, an email address, a phone number, or a certificate. Identification is a necessary but not sufficient control for preventing unauthorized or malicious access to a system or network, as it only declares who or what the user or system is, but does not prove it. Identification needs to be combined with authentication to verify the identity of the user or system. Data integrity is a control that ensures that the data is accurate, consistent, and complete throughout its lifecycle. Data integrity can be achieved by implementing various controls, such as encryption, hashing, checksum, digital signature, or backup. Data integrity can help to protect the data from unauthorized or accidental modification, deletion, or corruption that may affect the value, meaning, or usability of the data. However, data integrity does not prevent or detect unauthorized or malicious access to the system or network, as it only protects the data, not the user or system. References = CRISC Review Manual, pages 164-1651; CRISC Review Questions, Answers &Explanations Manual, page 952; What is Authentication? - Definition from Techopedia3; What is a Firewall? - Definition from Techopedia4