I think option C is correct because magma or lava is a molten or semi-molten material that is found under the surface of planet and volcano is the only source through which this molten material can come on the surface. For example, magma beneath the earth's surface come out through the vent of volcanoes present on earth's surface such as Mount Fuji, Barren island, etc. that's the reason why presence of magma over Venus's surface can explain the presence of volcanoes on the planet.
Because the Potassium ion loses an electron, the electrons come a little closer to the nucleus because they are attracted to the protons. This makes the ion smaller in radius