"They are not rocks."
Theoretically speaking, could it be grammatically correct to say chaHbe' instead of bIHbe'? I'm still only in the basic stages of Klingon, so I could be wrong, but I think if were talking about a group of beings capable of speech and how they're not rocks, you would say chaHbe'.
If you were pointing at a group of Klingons and wanted to say they aren't rocks, you'd say naghmey chaHbe'.
If you were pointing at a group of chairs and wanted to say they aren't rocks, you'd say naghmey bIHbe'.
When the course gives you They are not rocks and asks you to translate into Klingon with no further context, it should allow either translation. You have no way of knowing whether the sentence-writer was thinking of beings capable of using language or something else.
This is especially true in a Star Trek context, where sometimes beings capable of using language do, in fact, resemble rocks. Remember the Hortas and Excalbians? nagh ghaHbe' Hortavetlh'e'.
We would normally expect that the subject and object should match gender (here I mean grammatical type, not sexual gender), but there are times (especially with a negative like this) where they won't match - in which case, you probably wouldn't be faulted for choosing either option. So, theoretically, yes, there might be rare instance where you could use chaHbe' in a sentence like this. Except in this course! Here we expect you to imagine a context where they match. Unless we somehow make the mismatch explicit, in which case we would probably accept either answer.
If you were addressing a group of Klingons, could you also use the pu' suffix?
You've hit upon an area of grammar where we don't have much data: asking whether language-using nouns and non-language-using pronouns, or vice-versa, can be linked. We don't really know. To my knowledge, Marc Okrand has never used a sentence where this kind of mismatch exists.
For what it's worth, I think if you were pointing to a group of Klingons and wanted to say they're not rocks, you'd say naghmey chaHbe'. You'd use the generic, non-language-using plural suffix, because rocks don't use language. But you'd use the plural, language-using pronoun because you're using the pronoun to refer to the Klingons, not the rocks.