Which of the following best describes ransomware?

The description that best characterizes ransomware is that it is a program that locks users out of their system until a ransom is paid. Ransomware is a type of malicious software designed to block access to a computer system or files, typically by encrypting data, and demands payment to restore access. The essence of ransomware lies in its extortion model, whereby victims are coerced into paying a sum of money to regain access to their own files or systems, which they are otherwise unable to use.

In contrast, the other options do not align with the fundamental characteristics of ransomware. For instance, a malware that duplicates itself across networks refers to a different type of threat known as a worm, which spreads copies of itself to other devices. A harmless program that monitors user activities describes spyware, which collects information without the user's consent but does not lock out the user. Lastly, a type of firewall used to protect networks pertains to security hardware or software designed to prevent unauthorized access, which is entirely unrelated to the malicious intent associated with ransomware. Thus, the correct option accurately identifies the unique nature of ransomware as a threat to users and their data.