From your "Assignments" tab within any classroom, you can click on an assignment to be taken to the assignment results page. At the top-right of that page, you will see two buttons to export the assignment's data:
Basic: Simply exports the data you see in the view to a csv.
Detailed: Provides a very detailed breakdown of students, questions, and attempts that goes beyond what is shown in the view.
If you want to view how students answered incorrectly on multiple choice questions, download the Detailed (Excel) and explore the Students tab.