How do you access the Fediverse?
If you use more than one method, choose the one you use the most.
How do I determine this?
It's asking how you access the place you're posting on right now.
Is it through an app or the web? If it's an app, is it official (the one just called "Mastodon" in app stores) or third party?
If it's a Mastodon server in a browser, then the option would be "mastodon server website".
Some people use it through apps instead of browsers, and/or through non-Mastodon servers.
Excellent, thank you.