An incident response team has been notified of a virus outbreak in a network subnet. The next step should be to focus on limiting the damage by containing the virus and preventing it from spreading further. This may involve isolating the affected systems, disconnecting them from the network, blocking malicious traffic or applying patches or antivirus updates. Verifying that the compromised systems are fully functional, documenting the incident and removing and restoring the affected systems are possible steps that could be taken after limiting the damage. References:
* : [Incident Response Definition]
* : [Incident Response Process | ISACA]
* : [Virus Definition]

The best recommendation to prevent unauthorized access to a highly sensitive data center by piggybacking or tailgating is to use an airlock entrance. An airlock entrance is a type of access control system that consists of two doors that are interlocked, so that only one door can be opened at a time. This prevents an unauthorized person from following an authorized person into the data center without being detected. An airlock entrance can also be integrated with other security measures, such as biometrics, card readers, or PIN pads, to verify the identity and authorization of each person entering the data center.
Biometrics (option A) is a method of verifying the identity of a person based on their physical or behavioral characteristics, such as fingerprints, iris scans, or voice recognition. Biometrics can provide a high level of security, but they are not sufficient to prevent piggybacking or tailgating, as an unauthorized person can still follow an authorized person who has been authenticated by the biometric system.
Procedures for escorting visitors (option B) is a policy that requires all visitors to the data center to be accompanied by an authorized employee at all times. This can help prevent unauthorized access by visitors, but it does not address the risk of piggybacking or tailgating by other employees or contractors who may have legitimate access to the building but not to the data center.
Intruder alarms (option D) are devices that detect and alert when an unauthorized person enters a restricted area. Intruder alarms can provide a deterrent and a response mechanism for unauthorized access, but they are not effective in preventing piggybacking or tailgating, as they rely on the detection of the intruder after they have already entered the data center.
References: 1: CISA Certification | Certified Information Systems Auditor | ISACA 2: CISA Certified Information Systems Auditor Study Guide, 4th Edition 3: CISA - Certified Information Systems Auditor Study Guide [Book]

The metric that best indicates the effectiveness of awareness training is the number of users reporting receipt of the email to the information security team. This shows that the users are able to recognize and report a phishing email, which is a common social engineering technique used by attackers to trick users into revealing sensitive information or installing malicious software. The other metrics do not demonstrate a high level of security awareness, as they either ignore, follow, or forward the phishing email, which could expose the organization to potential risks. References: CISA Review Manual, 27th Edition, page 326