key word"Word documents, PDFs, and images no longer opening"

C. Crypto-malware

C is the correct answer Ann download the fine which seems like a malware. That does pop up and encrypted the file. A rootkit can be used to open a backdoor, allowing hackers into a system. Although rootkit can modify and have Full control over a system means that existing software can be modified, including software that might otherwise be used to detect or circumvent it. I will go with Crypto Malware.

WRONG!
The absolute correct answer is C. Crypto Malware.
Rootkit, spyware and backdoor are all stealth files, so there should be no reason for a popup. Also, the files cannot be opened, meaning they have been encryped.

I believe the answer is D. a rootkit.
A backdoor will not cause pop-ups or files to no longer open. That sounds like a file deny access, which rootkits are able to do. A backdoor should only allow hackers to return network or host they placed that backdoor to.