thanks for the response.
I already figured how to display an error message if an answer was missing.
However, my situation seems to be a little more complex. I could find a good tutorial for this yet.
Let's give an example:
The participant answers b. Perfect, then s(he) can continue.
The participant answers c. An error message will popup with "are you sure? please re-read the previous page". consequently, the participant will be redirected to the previous page and has to answer the questions again.
The whole page is a situation comprehension test after the participant has read a situation on the previous page. all answers need to be answered correctly in order to be elegible to do the survey.
How can I do that?