Answer:
I'd say A, B, C
Step-by-step explanation:
A, If you can't figure out what you're supposed to do, that'd be a valid reason to contact support.
B, If you can't register that sounds like a problem on their end and they should be notified
C, If you can't submit answers that'd be the same as B.
You wouldn't select D, that would be on you not the site.