6. Which statement best describes an igneous intrusion? A. Magma explodes to the surface and forms a smooth outer layer of rock. B. Igneous intrusions form when ice crystals splash into lava. C. Magma is exported to an open area and forms iron. D. Magma is injected into surrounding rock forming an igneous intrusion.

The correct answer is option D, that is, magma is injected into surrounding rock producing an igneous intrusion.

Magma refers to a semifluid or fluid substance within or underneath the crust of the Earth from which the formation of lava and other igneous rocks takes place by cooling. Igneous intrusion produces when the magma from beneath the Earth's surface is gradually pushed up to occupy the cracks or spaces between the rocks and solidifies and cools down prior of reaching to the Earth's surface.