Which security measure is commonly used to prevent unauthorized access to systems?

Firewalls are a security measure specifically designed to control incoming and outgoing network traffic based on predetermined security rules. They serve as a barrier between a trusted internal network and untrusted external networks, such as the internet. By monitoring and filtering traffic, firewalls help prevent unauthorized access to systems by blocking potentially harmful connections while allowing legitimate communication. This capability makes firewalls a fundamental component in safeguarding an organization's information systems and protecting sensitive data from being accessed by unauthorized users.

In contrast, spam filters address the issue of unwanted email and are used to enhance communication security but do not directly prevent unauthorized access to systems. Backup systems focus on data recovery and integrity rather than access control, while content filters are typically employed to restrict access to certain types of web content and may not provide the comprehensive security needed to safeguard systems from unauthorized access. Therefore, firewalls are the most effective measure within the given options for securing systems against unauthorized access.