Which of the following is an example of an extrusive igneous rock?

Basalt is an example of an extrusive igneous rock because it forms from the rapid cooling of lava that erupts onto the Earth's surface. When lava flows or volcanic eruptions occur, the molten rock cools quickly in the open air or in water, resulting in a fine-grained texture, which is characteristic of extrusive rocks. This rapid cooling prevents large crystals from forming, unlike intrusive igneous rocks, which cool slowly beneath the Earth's surface and can develop larger crystals.

In contrast, the other options represent different rock types. Slate is a metamorphic rock formed under pressure and heat, while limestone is a sedimentary rock primarily composed of calcium carbonate. Diorite is classified as an intrusive igneous rock because it forms from magma that cools slowly beneath the surface, leading to a coarse-grained texture. Understanding the formation processes and characteristics of these different rock types is essential in geology.