What choices, exactly, must one make for Medicare? If it were a test, how many questions would there be?

I suggest you read this and the other threads connected to it:

You’ll be vastly more educated when you get done. I know I was.

The very short version of your “What kind of test?” question looks like this to non-expert me:

You have one non-decision, one trivial decision, and a handful of decisions that each have dozens to hundreds of choices ranging from superb to disastrous with almost zero practical way to find the superb needle in the haystack of disaster & meh.

Good luck with that. You also don’t get a do-over on most of them.