A CO2 system could be a concern for an IS auditor when used to protect an asset storage closet. While CO2 systems are effective at suppressing fires, they can pose a significant safety risk to personnel. In the event of a fire,the CO2 system would fill the room with carbon dioxide, displacing the oxygen. This could be hazardous to anyone who might be in the room at the time12.
References: ISACA's Information Systems Auditor Study Materials1

The greatest concern to an IS auditor conducting an audit of an organization that recently experienced a ransomware attack is that backups were only performed within the local network. This means that the backups could have been encrypted or deleted by the ransomware, making it impossible to restore the data and systems without paying the ransom or losing the data. Backups are a critical part of the recovery process from a ransomware attack, and they should be performed frequently, securely, and off-site or in the cloud to ensure their availability and integrity.
The other options are not as concerning as option C, although they may also indicate some security weaknesses. Antivirus software was unable to prevent the attack even though it was properly updated, but this is not surprising given that ransomware variants are constantly evolving and antivirus software may not be able to detect them all. The most recent security patches were not tested prior to implementation, but this is a trade-off between security and availability that may be justified depending on the severity and urgency of the patches. Employees were not trained on cybersecurity policies and procedures, but this is a preventive measure that may not have prevented the attack if it was initiated by other means such as phishing or exploiting vulnerabilities.
References:
10: Infrastructure-as-a-Service Security Responsibilities - CloudTweaks
5: 3 steps to prevent and recover from ransomware | Microsoft Security Blog
7: How to Recover From a Ransomware Attack - eSecurityPlanet