c. at depths of about 50 to 200 km below earth's surface
Explanation:
Rocks begins to melt when sufficient temperature and pressure is applied on them.
To attain these conditions of temperature and pressure, a rock must be taken deep into the earth. With depth, the geothermal gradient of the earth increases. The geothermal gradient determines the temperature conditions a rock might exposed to deep within the crust. Rocks usually melt below the crustal levels and at the mantle.
The crust runs from about 0-100km in depth and the mantle takes off from there. We expect melting to fall beyond this region.
