Which of the following best explains how metamorphic rocks are formed?ARocks are changed by heat, with or without high pressure, deep inside the Earth.BWeathered rock is eroded, deposited, and cemented to form new rock.CMolten magma cools and hardens inside the Earth.DHot lava cools and hardens outside the Earth.

The best explanation for how metamorphic rocks are formed is A: Rocks are changed by heat, with or without high pressure, deep inside the Earth.

Here's a step-by-step explanation:

1. Metamorphic rocks start off as any type of rock, such as igneous, sedimentary, or even other metamorphic rocks.

2. These "parent" rocks are subjected to extreme heat and pressure deep within the Earth's crust. This can happen due to geological processes like tectonic plate movement.

3. The high heat and pressure cause the minerals in the parent rock to rearrange themselves, creating a new, denser rock. This process is called metamorphism.

4. Over time, these metamorphic rocks may be brought to the Earth's surface through geological uplift.

So, the key to the formation of metamorphic rocks is the transformation of existing rocks under high heat and pressure, not the cooling and hardening of magma or lava (which forms igneous rocks), or the erosion, deposition, and cementation of weathered rock (which forms sedimentary rocks).