Person C should have handled this situation more professionally than clearly trying to start a social justice fight on the company's Slack conversation. That's not an appropriate forum for her crusade, especially when the person making the comment about females was trying to provide support for winning the job in the US. If I were CEO I would pull her aside and tell her that she's entitled to her views, but she needs to be mature in how she engages with others. Person B I'd give a similar talk to, but far less stern, because he shouldn't have been lectured about the word "female" (which 99% of people don't consider to be "trans-exclusionary", but at the same time he shouldn't have fanned the flames with his aggressive response. I would also allow some time to pass, and look to push out person C for other reasons due to that person being a trouble maker. Those behaviors are destructive in company environments. If your company has published values and behaviors, ideally you could lean on those in your talks with the two employees.
Nice but then he/she/it sues you after her/his/herm/their HR complaint wasn't addressed in the manner she wants.. everyone who disagrees with her/his/herm/their to be fired.
The CEO should tell them both to grow up and work this out amongst themselves; then invite everyone to Treasures for Happy Hour so they can discuss how to focus their efforts into hiring more faggots and carpet munchers in order to truly diversify.
I assumed since the guy was discussing the cap table and who was on the ELT that they must have been executives themselves.