It is obviously B.
Draw a line from the corner of each plate and that is your minimum (flush).
Anything under is underfill.
More importantly - why have you got two plates offset like that ?
I'm with DekDee on this one. Check the mismatch/misalignment first, then worry about the underfill. I would also be concerned with the root side of the welded joint and the incomplete joint penetration resulting from the misalignment.
The thickness through the weld has to be at least equal to the thickness of the adjacent members provided the other criteria is within acceptable limits, i.e., misalignment, face reinforcement, melt through, etc.
