In this case the "M-" of "Mna" is a locative subject prefix. "There is" is translated into "Pana" for a definite place, "Kuna" for an indefinite place, and "Mna" for an interior place. The "M-" has nothing to do with the subject prefix of the 2nd person plural, but admittedly can be confusing.
It should be fruit unless you are making a point about there being a range of different kinds of fruit (and if that is the point, it needs to be signalled).
I have given up reporting this. They translate "tunda" as "fruit" and "matunda" as fruits, regardless of how we would use those English words. At least it illustrates how the Swahili words are used.
UPDATE: I went back to reporting all examples of "fruits" and I do think they have made good progress in accepting that "fruit" is uncountable in English. There are still some examples they have missed, and I have now reported this one too (4 Aug 2018).
I also tried "You have fruits..." but then I remembered that this is a Mahali nouns lesson, so I decided not to report.