Yeah, C and D make sense here. Debug lets you walk through the logic in Flow Builder and see exactly what happens, and sandbox means you aren't messing up real data while testing. Pretty confident that's what Salesforce pushes as best practice, but if someone thinks B fits too, let me know.
D . Debug in Flow Builder (C) helps catch logic errors, and sandbox (D) prevents production mistakes. B is tempting but running flows in prod isn’t really safe for initial tests. I could see some picking B, but I think C/D fits Salesforce best practices more-correct me if I missed anything.