"Cosa impari dal libro?"
I got this as the same multiple choice as you appear to have. And, similarly, I wonder why both are not correct. I went for "impari" but would have thought "imparo" (what did I learn from the book) is also correct.
Which both answers? ;) "imparo" would be first person singular, "What did I learn from the book?"
Here's another problem with duolingo. My comment was about a question, I believe a multiple choice type, but because the "main point" of that question (the preposition?) was the same as the one here, using a similar sentence, my comment appears here as well wherever I made it. I've seen this happen many times, and it can be a source of confusion (as here). I don't know if I was right since we are not seeing the question I originally commented on, but I was not talking about the translation of THIS sentence.