"We do not have fruit"
The answer should be " hatuna tunda" because " tunda" is singular for fruit.
No, not really. In English, "fruit" is generally an uncountable noun. It's normally only used countably when it means "a kind of fruit" (eg. "yoghurt with three fruits" means three kinds of fruit) but most of the time we just say fruit. If we want to specify number, we say "piece of fruit", so we can think of tunda as "(a piece of) fruit" and matunda as "(pieces of) fruit". Just because something is singular in English doesn't mean it has to be in other languages and vice versa.
That being said, because "fruit" is ambiguous about the number of pieces of fruit and it doesn't really matter when it's negative anyway because if you don't have "a cat", you definitely also don't have "cats", it should probably accept tunda too, if that is something Swahili speakers feel is idiomatic and equivalent to the English.