Now fixed (May 2018). Lots of other examples fixed too.

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.

Without "the" is more natural, actually, and "a fruit" would be wrong without some specific context to make it a countable noun (e g. "a fruit I have never seen before").

UPDATE: I assumed you were referring to "fruit". If you mean "seller", I totally agree.