How do teachers trace which students have viewed the assigned presentation or study material?

The teacher assigns a presentation or a study material (eg a PDF, DOC or PPTX file) to students via presentation.
How to assign students a file - a study material

Select the presentation in the "Results" tab.

Numbers next to the title of the assigned material indicate the number of students that have seen it / number of students the material was assigned to. This is the first information for the teacher.

Next, select "Answers by students".
At the top, we see that the student has seen 100% of the presentation. An "eye" displays next to each slide.

This feature works only for presentations.