Detailed
The next step after isolating the system is to perform a system scan to remove the malware (Option C). Since ransomware is suspected, running a comprehensive malware scan can help identify and remove the malicious software. It is crucial to deal with the active threat before taking further actions.
Scheduling an antivirus scan and system update (Option A) may help, but the immediate concern is identifying and removing the ransomware.
Educating the end user (Option B) is important but should happen after the immediate threat is resolved.
Creating a system restore point (Option D) would not be useful at this point since the system is infected.
CompTIA A+ Core 2 Reference:
2.3 - Detect, remove, and prevent malware, including handling ransomware.