The text under "Basics" lesson might help: "NOUN CASE
In High Valyrian, nouns change their form based on their grammatical role in the sentence. In this lesson, you'll be introduced to two cases. One you've been using since the beginning: the nominative case. The nominative case is used with the subject of the sentence. It's considered the basic form of the noun, and is the form you'll learn first when you learn a new noun.
The second case you're going to learn about in this lesson is called the accusative case. The accusative case is used with the object of the sentence. For example, in the English sentence "The man sees the woman", "the man" is the subject (the seer), and would take the nominative case in Valyrian. "The woman", on the other hand, is the object (the seeee), and would take the accusative case. In English, it's obvious who does what to whom, because a verb stands in between the two nouns. In High Valyrian, though, both of these are licit translations:
Vala ābre urnes. Ābre vala urnes. Rather than word order, the form of the noun is what tells you who does what to whom. In this case, ābra "woman" changes its ending from its usual -a to -e. There are several different strategies for forming the accusative case, so pay attention to each new form you see."
So abre for singular accusative and abri (with the long i accent) for plural accusative.
The pronunciation guide said that the subsequent noun would carry a different ending to indicate "and" before it. the choices gave us a singular aabra, a plural aabri, and a new noun ending, aabrii. Thus (and I got the question wrong) the correct word to make the two nouns make sense in the sentence would be aabrii.