Malware that locks your files and demands payment is:

The malware that locks your files and demands payment is known as Ransomware. This malicious software encrypts a user's files, making them inaccessible, and then demands a ransom, typically in cryptocurrency, for the decryption key. Its primary goal is financial extortion, leveraging the victim's need to recover their data. Ransomware attacks can be devastating for individuals and organizations, often leading to significant data loss or costly payments.

Let's examine why the other options are incorrect:

• Spyware (A) is designed to secretly gather information about a user's activities without their knowledge or consent, but it does not typically lock files or demand payment.
• Trojan (C) refers to malware that disguises itself as legitimate software to trick users into installing it. While a Trojan can deliver ransomware, its defining characteristic is deception, not the act of locking files for ransom itself.
• Worm (D) is a self-replicating malware that spreads across networks and computers without human interaction. While worms can carry various payloads, their primary function is propagation, not specifically file encryption and ransom demands.