What type of malware is designed to encrypt a user's data and lock access to their device?

Ransomware is a type of malware specifically crafted to encrypt a user's data, rendering it inaccessible. The primary goal of this malware is to extort money from the victim in exchange for a decryption key that would allow them to regain access to their files and devices. Typically, ransomware is delivered through phishing emails, malicious websites, or as payloads from other types of malware, and once it infects a system, it can quickly encrypt files on the device, often with little warning.

Adware, on the other hand, is designed to display advertisements, usually in an intrusive manner, and does not encrypt data. Spyware's function involves monitoring user activity and collecting sensitive information without the user's consent, which is distinctly different from data encryption. A Trojan horse masquerades as legitimate software to trick users into downloading it, but it does not necessarily encrypt data; rather, it can create backdoors or introduce other forms of malware.

Therefore, ransomware's unique characteristic of targeting data encryption for extortion makes it the correct answer.