I chose social.<name>.<tld>
But the important point for me is that the other two options have weaknesses. Anyone can register <name>.social before the organisation gets there, so you really want the domain name you already are on, for purposes of trust.
When it comes to Mastodon, you don't know that is the software you will necessarily always be using. So that one's out. 😁
But when it comes to which subdomain, not as important, Though I obviously like the one (social) you suggested 😉