Which grammatical case would the answer to this question be?
When "o" means "about" (so mostly), it takes Locative. Ta lekcja jest o poezji. (This lesson is about poetry).
czym o jest ta lekcja, is this possible?
It is impossible. This is the most unbreakable rule of Polish word order.
it is always preposition+ (words describing noun/pronoun)+ noun/pronoun
Even poets don't break this rule.
No, "o czym" is the only way round for this phrase. "O co" also exists, meaning "about what".
o co chodzi ta lekcja?
You'll need w+locative. O co chodzi w tej lekcji?
Would "O czym to ta lekcja?" have been accepted?
To only works with nouns on both sides.
OK, thank you