D was my other candidate before I noticed B lines up diagonally as well. Another thing I noticed just now about B is that all the corners of the center piece supplement the shape and colors it touches outside of it.
i.e. if you put B in the center, the top left shape is black triangle. To the left of it is a light grey triangle, diagonally there's a white triangle and upwards there's a grey triangle. All four colors are touching. This is the same for the other corners of the center B.
So that's four things going for it being B. I think I'm ready to place my bet.
