Astrid's computer screen suddenly says that all files are now locked until money is transferred to a specific account, at which time she will receive a means to unlock the files. What type of malware has infected her computer?

Ransomware

Explanation:

Ransomware is a special case of malware. It is a malicious program that infects a computer system by locking its screen and encrypting all its files and data and a certain amount of money (ransom) is requested from the owner of the system. Until the money is paid the screen remain locked and the files and data remain encrypted.

It is a very dangerous malware targeted towards organizations and even individuals. So systems that don't have strong security around them are easily attacked by this ransomware.

Note: A special type of ransomware is crypto malware or cryptojacking.