Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. ... When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly.

Explanation:
When lava cools, it undergoes a process called solidification, where it transitions from a molten state to a solid state. This cooling process results in the formation of rocks. The specific type of rock formed depends on the composition of the lava and the cooling conditions. In the case of lava, it is primarily composed of molten rock material, such as silica, iron, and magnesium.
The correct answer is "b. Igneous rock" because lava, when it cools and solidifies, forms igneous rocks. Here's a detailed explanation:
Igneous rock formation process:
1. Cooling: As lava cools, it loses heat energy and begins to solidify.
2. Crystal formation: During the cooling process, minerals within the lava start to crystallize or solidify. These minerals include quartz, feldspar, mica, and olivine.
3. Texture: The texture of the igneous rock depends on the rate of cooling. Slow cooling allows larger crystals to form, resulting in a coarse-grained texture. Rapid cooling leads to the formation of smaller crystals, resulting in a fine-grained or glassy texture.
4. Composition: The composition of the igneous rock depends on the original composition of the lava. For example, if the lava was rich in silica, the resulting igneous rock would be rich in silica as well.
Other options:
- Sandstone: Sandstone is a sedimentary rock formed from the accumulation and cementation of sand grains. It is not formed directly from the cooling of lava.
- Sedimentary rocks: Sedimentary rocks are formed through the accumulation and lithification of sediment particles, such as sand, silt, and clay. They are not formed directly from the cooling of lava.
- Metamorphic rock: Metamorphic rocks are formed from the transformation of pre-existing rocks due to high temperatures and pressures. They are not formed directly from the cooling of lava.
In conclusion, when lava cools and solidifies, it forms igneous rocks. This is due to the process of solidification and the crystallization of minerals within the lava.