It's so great for me that Polish is so similar to Russian. I really enjoy that
This is about the table = this is on the table? Or I understand na the wrong way.
I am confused where did you get "about" from. It doesn't suit this sentence at all.
It doesn't work that way. Usually the developers of the course can set translations for the word only once for whole course. That's why you get all 3 of them proposed, though in this case only one version is correct.
„Na” when talked about spatial relations means „na”. The others are used when phrase in Polish requires „na” but phrase in English requires another one. Not the best example: „For sure” = „Na pewno”. In this case „for” corresponds to „na”, but still it doesn't mean the same - however the meaning of both phrases is the same.