Answer:
Option C, At depths of about 50 to 200 km below Earth's surface
Step-by-step explanation:
The molten rocks with in the earth is found in the mantle layer. The upper mantle layer starts from 100 kilo meters to 200 hundred kilometers below the earth crust. The temperature and pressure at such depths is extremely high due to which the rocks starts melting. Even if the temperature is low , the excessive pressure leads to liquification of rocks.
Thus, option C is correct as from this depth onward the melting of rocks starts.