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").
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.