* Melting due to increased pressure: This is incorrect. Increasing pressure actually makes it harder for rocks to melt. Think of it like squeezing a sponge – the pressure makes it harder for water to escape.
* Melting due to increased temperature: This is the primary way magma is formed. Heat from the Earth's core or from the friction of tectonic plates can raise the temperature of rocks to their melting point.
* Melting due to the presence of water: This is also a common process. Water lowers the melting point of rocks, making them melt at lower temperatures. This often occurs at subduction zones where water-rich oceanic plates are pushed down.
Therefore, the answer is: Melting due to increased pressure.
