What signifies the cooling of magma in geological terms?

The cooling of magma in geological terms specifically leads to the formation of igneous rock. When magma, which is molten rock found beneath the Earth's surface, cools and solidifies, it crystallizes to form igneous rocks. This process can happen either beneath the surface, resulting in intrusive igneous rocks such as granite, or on the surface after a volcanic eruption, leading to extrusive igneous rocks like basalt.

The other options refer to different geological processes: sedimentary rock forms from the accumulation and compaction of sediment over time, metamorphic rock results from the alteration of existing rocks under heat and pressure, and fossil fuels originate from the decomposition of organic matter. Each of these processes involves different conditions and materials, and none are directly associated with the cooling of magma, which is definitively linked to the creation of igneous rock.