Which of the following best defines ransomware?

Ransomware is defined as a type of malware specifically designed to block access to a computer system or files until a sum of money, or ransom, is paid to the attacker. When a system is infected with ransomware, it typically encrypts the victim's files, rendering them inaccessible, and displays a message demanding payment for the decryption key. This form of malware exploits the urgency of regaining access to critical data, creating significant pressure on victims to comply with payment demands.

The other options don't appropriately describe ransomware. A network monitoring tool, while critical in cybersecurity, serves the purpose of observing network traffic for abnormal activity and does not involve any form of ransom. Encryption software generally aims to protect data by securing it, not to hold it hostage. Lastly, a firewall configuration is focused on controlling incoming and outgoing network traffic based on predetermined security rules, which is unrelated to the extortion nature of ransomware. Thus, the definition that highlights the malicious intent of extorting money by restricting access to data is the accurate representation of ransomware.