Which of the following processes primarily produces metamorphic rocks?

Metamorphic rocks are primarily formed through the process of intense heat and pressure. This transformation occurs when existing rocks, which can be igneous, sedimentary, or other metamorphic rocks, are subjected to high temperatures and significant pressures usually deep within the Earth's crust. This environment causes the minerals in the rock to recrystallize, change structure, and form new mineral compositions, resulting in a metamorphic rock.

For example, when shale is subjected to increased heat and pressure, it can transform into schist or gneiss, demonstrating how the original rock's properties can be altered under metamorphic conditions. Thus, the correct answer accurately reflects the geological processes fundamental to the formation of metamorphic rock.