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I don't see that option in the Dock preference pane.
That might be because it ain't in there. It's a preference for each folder on the Dock, therefore, each folder has that capability in its menu. Just click and hold the mouse over any folder and you get a fairly large pop up menu of items. One is quaintly named, "Show as: Folder" or "Show as: Stack." it appears that most of yours have "Stack" checked.