Igneous rock is formed when which material cools and solidifies?

Igneous rock forms when magma or lava cools and solidifies. While both magma and lava refer to the same molten rock material, the context in which they are found is different: magma is located beneath the Earth's surface, while lava is what we call magma once it erupts onto the surface. When this molten rock cools, it crystallizes and solidifies into igneous rock.

In this case, the correct answer is magma, as it specifically refers to the molten rock below ground that ultimately contributes to the formation of igneous rock upon cooling. This process is fundamental to the rock cycle and highlights the transformation of materials within the Earth's geology.

The other options do not correctly represent the materials that lead to the formation of igneous rocks: water is involved in other geological processes, such as sedimentary rock formation, while sand is a product of weathering and erosion and is not molten material.