Radioactive decay is the process by which unstable atoms break down and release energy in the form of heat. This heat provides an internal source of heat for the Earth, which is important in the formation of certain types of rocks.

Option 1: Igneous rocks are formed from the cooling and solidification of molten rock called magma. The heat from radioactive decay helps to melt rocks and create magma, which then solidifies to form igneous rocks. Therefore, the internal heat from radioactive decay is directly involved in the formation of igneous rocks.

Option 2: Sedimentary rocks are formed from the accumulation and compaction of sediment. While the heat from radioactive decay does not directly contribute to the formation of sedimentary rocks, it can indirectly play a role. The heat can cause the uplift and erosion of rocks, which leads to the creation of sediment that eventually forms sedimentary rocks.

Option 3: Metamorphic rocks are formed from the alteration of pre-existing rocks by heat and pressure. The internal heat from radioactive decay can provide the necessary heat for the metamorphism of rocks, leading to the formation of metamorphic rocks.

Alert - correct answer should be "4. All of the above". The internal heat provided by radioactive decay is involved in the formation of all three