The picture below shows metamorphic rocks on the earth's surface.

Referring to the attached image below which represents the complete rock cycle, we will find that:
The metamorphic rocks are formed due to heat and pressure applied either on igneous rocks or on sedimentary rocks.
This means that heat and pressure are the main factors in the formation of metamorphic rocks.

Based on the above, the best choice would be:
a. extreme changes in temperature and pressure

My answer is opptionc, just check with anyone else

Independent variable is something that is changed in a experiment.