Igneous rocks are commonly classified by where cooling occurs.

• A. Clastic and crystalline
• B. Weathering and erosion
• C. Regional and contact metamorphism
• D. Plutonic (intrusive) and volcanic (extrusive)

Answer: (D)

Igneous rocks are distinguished by where their molten material cools. If magma cools slowly underground, crystals have time to grow large, forming intrusive or plutonic rocks. If lava erupts onto the surface or near it and cools rapidly, crystals stay very small or the rock becomes glassy, forming extrusive or volcanic rocks. This cooling-location distinction is exactly what the terms intrusive/plutonic and extrusive/volcanic describe, which is why that choice is the best fit. The other options refer to processes or rock types not about how the igneous rock originally cooled—sedimentary textures (clastic vs crystalline), surface-weathering processes, or metamorphic changes due to heat and pressure.