Agraywacke is

a. a limestone with abundant, sand-sized, quartz grains.
b. a sandstone with the sand grains embedded in a clay-rich matrix.
c. a dark-gray, calcite-rich mudstone or shale containing pyrite.
d. a dark, organic-rich, chemical sedimentary rock containing small crystals of halite.

The correct answer will be option-B

Explanation:

Graywacke is an impure sandstone which is characterized by its dark color, hardness, poorly sand-sized quartz grains mostly embedded in the muddy matrix made of clay.

Graywacke is widely spread in the Earth's crust and constitutes about 20-25% of the total sandstones on the Earth.

The graywacke is widely used by humans for the construction purpose of road and sea walls.

Thus, Option-B is the correct answer.

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