Final answer:
Metamorphic rocks are primarily exposed at the surface in current or former mountain belts, continental shields, and regions of deeply incised crust due to processes like mountain building and erosion, which bring these rocks to the surface.
Step-by-step explanation:
The metamorphic rocks are predominantly exposed at the Earth's surface in all of the above options: 1) current or former mountain belts, 2) continental shields, and 3) regions of deeply incised crust. These areas provide the conditions necessary for exposing metamorphic rocks, such as deep burial and tectonic pressures that lead to uplift and erosion.
Mountain belts feature high temperatures and pressures from tectonic collisions, which facilitate the formation of metamorphic rocks. These collision zones often present a predictable pattern of metamorphism known as the Barrovian sequence, where rock grade increases toward the center of the collision. As erosion wears down the mountains over time, it exposes these high-grade metamorphic rocks.
In continental shields, ancient metamorphic and plutonic rocks are often found at the surface. These shields represent the stable interiors of continents, where deep crustal rocks have been exposed by extensive erosion over geological timescales.
Deeply incised crustal regions expose metamorphic rocks through significant removal of overlying material by erosion, often associated with uplifting forces.