This is such a challenging thing for English speakers, because we would not say "These are fruits" in common English. We would point to a bunch of fruit and say "That is fruit." But it makes sense in Swahili, so I am not sure how to fix it. Perhaps English speakers just have to adjust for the purpose of translation here?
I think "this is fruit" should actually be the correct translation here. Matunda is often translated as "fruits" in this course, but in English fruit is almost always used as a mass noun, so "fruits" sounds odd (except when talking about different kinds of fruit, or in fixed expressions like "the fruits of one's labor").
I would be in favor of the english being written as "this is fruit (pl.)" when it is a prompt and when the prompt is in Swahili both "this is fruit" and "these are fruits" could be acceptable. Or use a picture.
What do you have to use if you're referring to something unspecified? Is it the n/n class?
Something unspecified as in what? Like if you're asking "What is this?" I always used n/n class as you said. It's also acceptable to use vi- class, as if you are asking "what is this (thing)?" It doesn't matter a whole lot, but those are the two I would commonly use in speech.
Demonstratives are always specified (you cannot talk about this or that unless it is known/specified). If you do not know the class of what you are asking for, as rokksolidrees said. ;)
Fruit is often treated as a mass noun in English, like water, for example. One would not say, "These are water," or "These are waters," unless maybe referring to a bunch of bodies of water. Reported.