Choices column is now supported in Portal and is GA in 2021 Release Wave 2 from Feb 15, 2022:
However, you may find it not working, and there is no official document about how to enable it.
The solution is to create a Site Setting record to enable the feature:
And that’s it! It should show up on Portal like this:
To learn more ways to configure it on your Basic Form / Advanced Form / List view, visit here: