Yep. The singular is tunda.
The issue you're probably having here, reading through the lines a bit, is that we rarely use "fruits" in English except to talk about different types of fruit, or in some special contexts and phrases. It's generally uncountable. So you can think of it like this:
tunda = a piece of fruit
matunda = pieces of fruit
If it doesn't accept "fruit", that's an error with the course and will eventually be fixed for sure.