A. a layer of granite
just took the test and got it right :)
A layer of granite
An igneous rock which is formed by the cooling of a molten body of rock and more cooling of the molten body of rock (magma) is a Granite. The surrounding rock is more dense than the the body of molten material and they rise in form of bulb shaped known as plugin.
After rising, the granite slowly cools down, visible crystals that makes up layers are formed during the slow cooling process.
Water hardly penetrate through a layer of granite because of it's dense nature. The layers of granite prevent the vertical movement of water.
In this case water pass through the layer of rock because it is less dense, but it can't pass through a layer of granite because it is very dense and if forms a bulb(onion) like shape.
The answer is; a layer of granite
This layer so granite is impervious hence is not porous. Due to its non-porosity, water cannot pass through it. These types of rocks are important in aquifers. When an impervious rocks sandwich a pervious layer or are below a pervious rock, an aquifer is created.