Which of the following is a type of malware that prevents the system from being used until the victim pays the attacker money?

Ransomware is a type of malware specifically designed to restrict access to a computer system or its data until a ransom is paid. It typically encrypts files or locks the user out of their system, rendering it unusable. The attackers then demand payment, often in cryptocurrency, to restore access to the victim. This method exploits the urgency and fear of users who may need immediate access to their data or systems, thus coercing them into paying the ransom.

In contrast, a fileless virus operates differently by executing malicious activities in the computer's memory rather than relying on files stored on the disk, which does not directly lock users out of their systems or demand payment. A denial-of-service attack overwhelms a system with requests to make it unavailable but does not typically involve financial demands tied to regaining access. Remote Access Trojans enable attackers to take control of a system remotely but do not inherently cause a lockout that would lead to ransom demands. Thus, the defining characteristics and operational mechanics of ransomware clearly establish it as the correct answer.