Which of the following is used to prevent unauthorized access to a computer system?

A firewall is a crucial security measure designed to prevent unauthorized access to or from a private network. It acts as a barrier between a trusted internal network and untrusted external networks, such as the internet. Firewalls can be configured to block or allow data traffic based on predetermined security rules. This capability to filter incoming and outgoing traffic makes firewalls essential for protecting sensitive data and maintaining the integrity of computer systems.

In contrast, malware refers to malicious software that can compromise system security rather than protect it. Antivirus software is designed to detect and eliminate malware threats but does not necessarily control access to the network itself. Backup software is used for creating copies of data to prevent loss rather than safeguarding against unauthorized access. Each of the other options serves a distinct purpose within the realm of cybersecurity, but a firewall specifically is focused on access control, making it the appropriate choice for preventing unauthorized access.