It's all of them.
Tony Chocolonely's whole image is to be as fairtrade as possible and even they admit that they can not guarantee no forced labour was used in their production chain.
They could easily insource their entire operation in order to guarantee slave free chocolate, but you would end up with very expensive chocolate. That is probably why they dropped their "100 percent slave free" slogan. They still need to make a profit and at the moment you can only do that with slave labour.
