Driver B, no question about it. Driver A took the inside line and he had it, but driver B turned into driver A, and from the looks of it, driver B knew that A was there. If driver B would've held his own line, that pass should have been clean and fair. But not only did he almost take out driver A, after the incident he spun around just after the chicane, hitting another car. Not so harsh, and nothing that seemed to cause alot of damage, but it still seems as he has a huge lack of understanding the situation and how to solve it in the best way. It doesn't work to just spin around, without watching mirrors/minimap first. Clearly he didn't check the minimap, nor the mirrors (I can understand the mirrors though, they wouldn't have helped out much in that position). Either that or he did check the minimap but just didn't give a dime about the cars coming in. In this case we can't do anything but speculate.
The conversation afterwards makes driver B look very childish. There is room for two cars in that chicane, if both cars are holding their lines. Of course only one car fits in that chicane when driver B did what he did. Duh.
So, conclusion: Driver B is held responsible, imo.