Which type of rock is formed from the hardening of molten rock?

Igneous rock is formed from the hardening and solidification of molten rock, known as magma when it is underground and lava when it erupts on the Earth's surface. This process occurs when molten material cools down, leading to the crystallization of minerals. The characteristics of igneous rocks, such as their texture and mineral composition, are largely influenced by the rates at which the molten rock cools. For instance, when cooled quickly on the surface, such as during a volcanic eruption, igneous rocks tend to have small crystals (fine-grained). When magma cools slowly underground, larger crystals can form (coarse-grained).

Sedimentary rock is formed from the accumulation and compression of sediments, which is distinctly different from the formation process of igneous rock. Metamorphic rock arises from the transformation of existing rock types due to high pressure and temperature changes over time, which also does not involve molten rock. The term "mineral rock" is not a standard classification in geology, as rocks are generally categorized as igneous, sedimentary, or metamorphic based on their formation processes.

Therefore, igneous rock is the correct answer because it is unmistakably linked to the hardening of molten rock.