Final answer:
The volume of rock increases when it heats and liquefies into magma due to thermal expansion, making the correct answer C) Increases.
Step-by-step explanation:
When rock heats and liquefies into magma, its volume typically increases. This is because heating a solid rock causes its mineral structure to break down and transform into a liquid state, which generally has a larger volume than the solid because of the thermal expansion that occurs when a substance is heated. The correct answer is C) Increases. This volume expansion happens because heat causes the particles within the rock to move more vigorously, pushing them further apart.
For instance, conditions such as an increase in temperature (arrow "a") or a decrease in pressure (arrow "b"), as per the crustal rock's Pressure and Temperature graph, can induce melting of the rock into magma. Adding water can also change the melting conditions by shifting the solidus line to the left (arrow "d"), thus leading to the production of magma.