Final answer:
An Intrusion Detection System (IDS) would be most useful for Henry in preventing future cyber-attacks, as it proactively monitors for malicious activity and policy violations, complementing other security measures such as firewalls, antivirus software, and encryption.
Step-by-step explanation:
After a major cyber-attack on his company, Henry is looking for the best method to prevent future attacks. Among the options listed, an Intrusion Detection System (IDS) would be most useful for this purpose. While firewalls and antivirus software are important lines of defense, they are more reactive and focus on preventing known threats. An IDS, on the other hand, proactively monitors network traffic and system activities for malicious activity or policy violations. If an intrusion is detected, an IDS can take action, such as alerting administrators or automatically responding to the threat.
Moreover, encryption is essential for protecting data in transit or at rest, making it unreadable without the correct cryptographic key, but it does not inherently prevent intrusions. Therefore, a combination of these security measures, including using a cutting-edge IDS, would significantly fortify Henry's cybersecurity posture against future attacks.