The 5th item is about "niet" and "geen" specifically. I think that often "niet" is used to negate the whole sentence or thought or the verb. The object is then complementing the verb and "niet comes after it and even any adverbs as well, but before any second part of a verb which would be at the end of a sentence. When you want to negate a particular adjective, adverb or noun that is used with a definite article then "niet" comes before the item to be negated in which case you are stressing NOT that item being negated but some other - yes.
Please scroll down to see all the examples and you will see some with pronouns used as direct objects and even a noun used as a direct object. Naturally, if there are no adjectives, adverbs or nouns after the verb, then "niet" is seen right after the verb and that happens to be the end of the sentence. https://www.duolingo.com/comment/3734833
my guess is that you were doing a listening exercise. If that's the case, it has to do with the different pronunciations of -e and -ij in pronouns. The voice says zij, and so, ze is incorrect, as the idea is that you get to work on the difference (in terms of pronunciation) between these two words.
Hope this helps.