Which of the following best describes a firewall?

A firewall is primarily designed to prevent unauthorized access to or from a network. It acts as a security device, monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. By filtering traffic, a firewall helps to protect sensitive data and maintain the integrity of the network infrastructure. This is crucial in defending against cyber threats such as hackers and malware, making it an essential component of network security.

The other options do not accurately reflect the function of a firewall. For instance, while a physical barrier can protect hardware, it does not encapsulate the definition of a firewall. Similarly, enhancing Wi-Fi speed or increasing battery life are not relevant to the role of a firewall, as those functions are associated with performance optimization rather than security measures. Therefore, the description focusing on preventing unauthorized access best encapsulates what a firewall is and its role within network security.