I looked into this and...assuming you mean the assessment for Unit 1 - Chapter 1, where question 9 is a hex conversion question, then the correct answer is marked as C in the system right now.
Is there a chance you students did this assessment a while ago (like back in the Fall) and you're having them revisit it now to review? I'm wondering if we should look into whether this question was marked incorrectly in the past, somehow got locked in, and your students' accounts are just not responding to the new(er) correct answer.
Any information you can provide will help, but I'll have to bubble this up to our engineering team. Thanks and sorry. We'll figure it out.