"Where did you buy these hotdogs?"
Translation:Де ви купили ці сосиски?
«Купува́ти» (with its forms ти купува́в, ви купи́ли) is the imperfective verb, it presents an action as something that has a beginning and an end. It's used to emphasise that the action was in progress for some time (if you know the listener spent some time buying the carrots), or for a repeating action (But if it's a repeating action, why are you using the present tense? It works only if you know the listener bought carrots at one place for some time and then stopped doing that).
«Купи́ти» (ти купи́в, ви купи́ли), on the other hand, is a perfective verb. It presents an action as something that happens immediately. The action denoted by the word «купи́ти» happens when you finish buying the carrots. You use this verb when the action is not repeating and when you don't want to emphasise the duration of buying the action.
In most contexts, «Де ти купи́в мо́ркву?» sounds much more natural than «Де ти купува́в мо́ркву?».
I remember, when I lived in Kyiv to say "купував" was absolutely normal. But, probably...
Please don't make a one-to-one projection of English onto Ukrainian and back :) The grammatical structures I mentioned are not equivalent, I just wanted a short answer... If it's not so clear, here goes a more detailed one :)
Ukrainian language doesn't have concepts of continuous tense and simple tense. We don't even think in these terms. But we do have the concept of verb aspect. There are two: imprefective ("unfinished") and perfective (finished).
The verb купували does not imply continuos tense (i.e. being in the process of doing an action). It implies doing the action but not necessarily finishing it. It is unknown whether it was finished or not. Very often it implies that it wasn't finished, since this aspect of the verb was preferred in the first place, but not always. Example: Ви купували сосиски, але не купили.
Я мила посуд, але не помила. "I was washing the dishes, but didn't wash them" wouldn't really work. It's more like, "I washed the dishes, but didn't finish washing them." Or actually the meaning could be "I washed the dishes but didn't do a good job at it so although I called them washed effectively they are not washed".
Also: In this particular question it is not important which verb the speaker uses, купували or купили, since both of them, as mentioned, can represent a finished action. One can chose either aspect to ask about where the sausages were bought, because the information being requested is "where" and not "did you actually buy them finally"
So I would go with allowing either translation here. This is not a strict grammar textbook exercise, it's more about using the language to convey your meaning.