What question? and What is the question? are different things. The first one is asking to specify which question it is they’re talking about among a few options. The second is asking you to say the question.
Remember that there is no copula in Turkish, so there is another word for which, hangi, to avoid such confusions.
lingonerd, please read the comment of Orde90 and Ladynurington at the beginning of this discussion. And have a look at Ektoraskan's explanations here: https://forum.duolingo.com/comment/8649151/Grammar-Suffix-dir.
the n on the end of soru makes it "your" so it becomes a very loose term soru nedir " what's the problem" to "what is your problem?" (or question) the dir on the end of words emphases it as sort of a fact or definite statement. For example: If Turkish people had the saying, "that is the last straw!" it would sound like, son pipetDIR. It is just emphasizing a statement, that we do with stress of our words, such as Americans always use stress of words instead of actually having a word for it. For example, to define what you are asking a question about in Turkish, depends on the placement of mi? If I ask, "Is THAT GUY from Italy? It would be written as O adam mi italyali? But if I ask, "Is that guy from ITALY?" The question would be written as, O adam italyali mi?
In written Turkish as this Duo example it is stressing the importance of the question being asked. "Soru nedir?" In English it means "is." You will encounter these suffixes again on Duo as written Turkish & recognise the correct suffixes to apply.
The Suffixes DIR / DİR / DUR / DÜR / TIR / TİR / TUR / TÜR
Last Vowel Ending / Suffix:
a, I - dır
e, I - dir
o, u - dur
ö, ü - dür
If the word ends in one of these consonants:
[f, s, t, k, ç, ş, h, p] the suffixes are tır, tir, tur, tür
Mel.ancholia, "what was the question?" is wrong because "nedir" = "what is it?" is at the simple present tense. "What was the question? , past tense, would be "Soru nedi?". Have a look there: https://www.turkishexplained.com/tobepos.htm and also on Duolingo Turkish Grammar Portal for the verb to be at the simple present tense: https://forum.duolingo.com/comment/7738500 bu AlexiNotTurkey.