A customer is unable to open some files on their system.

Comprehensive and Detailed Explanation From Exact Extract:

Ransomware is a type of malware that encrypts the user's files and demands a payment (ransom) for the decryption key. When a user receives a message stating that their files are encrypted and cannot be accessed, ransomware is the most likely cause. The attacker’s goal is to hold the data hostage until the victim pays to restore access.

A. Keylogger records keystrokes and doesn’t encrypt files.

C. Phishing is a social engineering tactic to gather credentials, not to encrypt data.

D. Cryptominer uses system resources to mine cryptocurrency, not encrypt files.