Answer:
C. Sandstone
Explanation:
Magma is a liquid, so that means it can't be that.
Volcanic rock is hardened lava, which means it can't wear away that quickly so that means it can't be that.
Granite is a solid sedimentary rock that settles over several thousand years, which means it can't be that.
That leaves sandstone which is made up of sand, hence the name sandstone, which is inherently easy to move which would make it the quickest to wear away.
