Final answer:
The majority of igneous rocks, about 90%, are associated with plate boundaries, where tectonic activity leads to magma production and the formation of new igneous rock.
Step-by-step explanation:
The question asks which of the provided options is most commonly associated with 90% of igneous rocks. The correct answer is: a. Plate boundaries. Igneous rocks form through the cooling and solidification of molten rock, or magma. On Earth, the vast majority of igneous rocks are associated with plate tectonic boundaries. These regions are characterized by high levels of seismic and volcanic activity, where mantle convection and plate movements result in the production of magma.
At divergent boundaries, such as the Mid-Atlantic ridge, plates pull apart and magma rises to fill the gap, forming new igneous rock. At convergent boundaries, which include subduction zones, a dense oceanic plate descends beneath an overriding plate, often creating magma that feeds volcanic eruptions. Although significant accumulations of igneous rocks can occur in areas with mountainous relief due to subduction-related volcanism, as in the western margin of South America, the majority of igneous rocks are linked to the overall activity along various plate boundaries, both divergent and convergent.