Of course, I don't know as much as Yah-E in this catergory, but I'm also going to go with mantle dentin. The limited knowledge I have makes this seem like a correct decision, because the mantle dentin is deposited peripherally in the dental papilla beneath the enamel organ. Like Yah-E said, this happens at the onset of dentin formation. Seems like a wise choice.
By the way, what is the purpose of this question? It seems as though you have an answer in mind and are baiting us towards it.