Concept:
Metamorphic rocks are formed when existing rocks are changed due to heat, pressure, or chemically active fluids.
Step 1: Granite is an igneous rock.
Step 2: Coal is generally considered an organic sedimentary rock.
Step 3: Sandstone is a sedimentary rock.
Step 4: Slate is formed from shale due to metamorphism.
Step 5: Therefore, the metamorphic rock among the given options is:
\[
\boxed{\text{Slate}}
\]