The correct answer is J: Cooling rate of molten rock.
1. The rate at which molten rock, or magma, cools determines the size of grains in igneous rocks.
2. When molten rock cools slowly, typically underground, larger grains have time to form as the minerals have more time to grow, resulting in coarse-grained igneous rocks like granite.
3. In contrast, rapid cooling, such as when lava cools quickly on the Earth's surface, leads to the formation of fine-grained rocks like basalt with smaller grains.
4. Therefore, the cooling rate of molten rock plays a crucial role in determining the grain size of igneous rocks.
