The correct answer is option a, that is, sedimentary rock.
There are three main kinds of rock on Earth: igneous, metamorphic, and sedimentary.
The sedimentary rock envelopes about three-quarters of the land on the surface of the Earth. However, sedimentary rock forms only about 5 percent of the top 16 kilometers of the crust of the Earth. The other 95 percent comprises of metamorphic and igneous rocks.
Sedimentary rock is malleable of the three kinds of rock. Thus, it is the quickest to erode.