The correct answer is - magma intrusions.
The metamorphic rocks are rocks that basically have been either igneous or sedimentary, but have changed under certain circumstances. The initial igneous or sedimentary rocks, because of the geologic activities, often end up deep into the ground. As they get deep into the ground, they are exposed to higher temperatures and higher pressures, which over time change their structure, thus the creation of the metamorphic rocks. Also, the intrusion from the molten rocks known as magma, influence the creation and the type of the metamorphic rocks. This influence can be expected at places that are close to the plate boundaries, and where there's active volcanic activity in the region.