I agree with you in that a single word on its own does not form a sentence, but you can kind of think of nominative case as the "default" case. When no other "clues" exist to help you identify what a word's relationship is to other words, it is common convention/practice to present a word in its most basic form -- the nominative case. Adding suffixes to a word to indicate its case is something that you do when you know the word's relationship to the other words in the sentence. Without other words to reveal what role in a sentence a word is playing, the assumption should be made that the word should be presented in its most basic form. Similarly, a dictionary does not present the declined forms of a word first; it presents the most basic form.
If you've studied math at all, a word without any other words to put it into context is kind of like the absolute value of a number. A number can be positive or negative, and you can add a "+" or "-" sign to specify , but its absolute form is the number itself without additional demarcations.
|-10| and |+10| = 10
Applying this to this example, the nominative (or absolute) form has been bolded:
Also, just in general, it's best to try to not get too fancy when answering these prompts, if you want to get a correct answer. Duolingo isn't like one of those tests that tries to trick you by having the least likely answer be the correct one. Though some examples have been a bit trickier than others (e.g., "Evet, domatesi ve yağı."), most of the time, duolingo is really just testing to see if you can translate a word or phrase in a basic, uncomplicated way.
Hope that helped.
It's an understandable question, but it's worth knowing that a word in the accusative without any other context does not make sense. The accusative case is a way of marking a noun that is receiving some kind of action. The action has to be mentioned in order to use the accusative.
It's coincidence, but note also that K is a lot more common in Turkish than in English. It is the 4th most common consonant after R, N and L and the 7th most common letter in Turkish. On the other hand, K is the 17th most common consonant in English and the 22nd most common letter.