Generally on AWD vehicles if you replace one tire you need to change them all as the larger diameter on the new tire (due to less wear ofc) can play havoc with your drivetrain and traction control etc.
That said looks like there is quite a bit of tread on these tires, might be able to get away with replacing just one.
Better to plug it though.
@Exterous what even is that though? Looks like you drove over a pencil or something, but I feel like that shouldnt be able to do that kind of damage to your tire.