Which igneous rock is formed at the Earth's surface?

Basalt is an igneous rock that is formed at the Earth's surface through a process known as extrusive igneous activity. When magma (which is molten rock beneath the surface) reaches the Earth's surface, it erupts through volcanic activity and rapidly cools, solidifying to form basalt. This rock is typically characterized by its fine-grained texture, which occurs because the rapid cooling doesn't allow for large crystals to form.

In contrast, granite and gabbro are examples of intrusive igneous rocks, which form from magma that cools slowly beneath the Earth's surface, leading to the formation of larger crystals. Intrusive rock is a broader category that encompasses these rocks formed underground but does not specify a particular type like basalt does. Therefore, basalt is the correct response as it specifically represents an igneous rock formed at the Earth's surface.