Answer: Troubleshooting
Step-by-step explanation:
Based on what I know, system diagnostics are just trying to find out what the problem exactly is, and looking for what exactly is wrong with your computer. TCP/IP is just a transmission protocol for how data is transmitted in the internet. Virus scanning is just a part of system diagnostics, so see what exactly is wrong. None of those other options seem to be somehting that contains an actual trial and error. Troubleshoorting is to solve a problem, and solving problems can take trial and error. For that reason, I would say troubleshooting.