"Will you buy a carrot" is the only option I can choose.....I never buy one carrot!
'some carrots', treating 'marchewka' as kind of a mass noun, is a starred option as well.
Well... on one hand, I never thought of "marchewka" as a mass noun (there is a word that is definitely a mass noun - "marchew" (declension here), but I think my mother is more likely to say "Kup marchewkę!" than "Kup marchewki!", and of course she won't mean a single one. English will almost definitely use plural, though.
Well, my mother is Polish and she does say "kup marchewki". Maybe it's a family thing :P
I checked out of curiosity and found out that "marchewki" wasn't accepted so far, added it of course.
I've tried using "Are you buying ...?" with both a carrot and carrots, it's not accepted. I assume "Will you buy ...?"is demanded. I of course don't yet have the feel for subtle differences in Polish, therefore this question. For me, "Are you buying ...?" is more neutral than "Will you buy ...?", which (for me) has a stronger undertone of a request. Are there similarities in Polish?
Basically, we want the students to feel what is 'Present Continuous in the future meaning' and what is the real Future Simple. So we don't accept them interchangeably.
Moreover, in this sentence... "Are you buying" looks even more different than usually to me. This question makes most sense for me as a request.
"Are you buying carrots?" is asking someone if he/she is right now in a proces of buying carrots and have nothing to do with future.
"Marchew" is the name of this plant species (https://pl.wikipedia.org/wiki/Marchew_zwyczajna), it can be singular as well. And in my understanding "marchewka" can be a mass noun. Especially if you imagine that it is already sliced or grated. "Marchewka z groszkiem", being a dish, is definitely uncountable :)
I understand the sentence "Kupisz marchewkę?" as it was uncountable, like "kupisz kilogram marchewki?". Personally I would specify if I wanted to buy one carrot: "Kupisz jedną małą marchewkę?"
"Kupować". So theoretically you can say "Będziesz kupować/kupował/kupowała marchewkę?" (Future Compound), but that would be rather strange in most situations... (Will you be buying carrots?)