Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from shale through a low-grade regional metamorphism process. The original shale deposits were subjected to heat and pressure, causing them to transform into slate. This transformation process involves the recrystallization of the minerals in the shale, resulting in the formation of a dense, fine-grained rock with excellent cleavage. Therefore, the correct option is shale.