Translation:They are buying something at the supermarket.
I have a different opinion. You can be right in certain context.
Most likely, we say this when those people are currently shopping in the supermarket. But we can also say this to talk about where they usually go for their grocery.
Where do they usually go for buying things.
They go to the supermarket for buying things.
How about you?
I don't go to supermarket for buying things. I would go to the small shops around here.
(The translation is for better understanding only, not good English.)
To indicate more precisely they are buying things now, we can say:
Yes but no, it reads that they are at the supermarket... Chinese is that confusing, you can be right as much as you can be wrong... And there are so many Chinese languages within China that it can be quite confusing among themselves as well... Believe it or not... Here we are fighting for a reasonably acceptable and Duolingo seems to have a hard time digesting all suggested answer.. They really have to employ more people.... Create jobs.... Could be a very positive option.
What is really frustrating is that the lessons offer practically NO help before you start taking the "test". I don't like the "hit-or-miss" approach. I can sense that it is not about actual translation but how a Chinese person would say something, versus an English translation verbatim, but how about some instruction beforehand!
Don't the tips they offer help? But I do know the problem of the many ways of expressing the same or similar things when translating into English. It would be impossible to cover all of them. I'm OK with accepting their answers even if they don't accept mine, unless their English is actually wrong. At least I can assess it according to my own use of English and feel I've learnt something. I make sure I get their version of right next time!!! Often it's a problem of American versus English or all the other versions of the language. And that helps make it all interesting.
In the U.S., grocery store and supermarket are often used interchangeably.
The characters used (超市) are the abbreviation for 'supermarket'. I looked up 'grocery store', the characters are '杂货店' (zá huò diàn) (mixed products shop). So you can say 'grocery store' and still be correct. Since they used the specific characters for 'supermarket', that is what they will expect.