The correct answer is (c)The rapid release of numerous gas bubbles during cooling.
Pumice is created when super heated highly pressurized rock is ejected from volcano. The foamy nature of pumice is due to continuous cooling and depressurization. This depressurization creates gas molecules in pumice. When the lava is hot many gas molecules are present in it and the cooling of lava under high pressure leads to eruption of these gas molecules. This makes the pumice stone porous.
