Right, I think I have the source of confusion. It's partly my fault for not checking this, but part of the question is missing and I didn't look at this closely enough. My fault.
Here we go again.
In a Hall slice it depends on whether the charge carriers are positive or negative. (They are often positive/semiconductor)
If the field is into the page, and conventional current is to the right, as in the diagram, the force on the charge carriers is upwards.
If the charge carriers are electrons, the top becomes negative. If the charge carriers are positive, the top become positive.
In the question the carriers are, I presume, electrons.
Sorry for the confusion.
I've modified my earlier reply so as not to confuse others reading this from the start.