Lava that cools quickly forms rocks.
a) extrusive metamorphic
b) extrusive igneous
c) intrusive metamorphic
d) intrusive igneous

The correct answer is - B) extrusive igneous.

The lava is essentially a magma that managed to reach the surface of the Earth. It can cool off quickly or slowly, depending on the conditions of the surrounding area. When the lava does cool off quickly, the result of it are the extrusive igneous rocks. The quick cooling of the lava is caused either by it being on the surface of the Earth, which is much cooler than its temperature, or if it comes in contact with water.

The word extrusive, basically means ''on the outside'', so it is a nice indicator of where the cooling has been taking place. With the quick cooling, the igneous rocks that form, do not have enough time to create big crystals, instead they have very small crystals, almost unnoticeable. Some rocks that form in this way, are the basalt, andesite, and obsidian.