I did not get the choice to attend option A of the assessment day, that is strange. I live in Spain BTW so the time zone is really not a constraint.
I'll try to ask about what specifically is done on the assessment day, but what I know is that they test your ability to work with others in an academic environment. My guess is that it is a case study, but who knows...
It used to be something to know more about candidates who the adcom had doubts about, but I guess it will be another hurdle in the admission process now.