The question asks which of the given options is NOT a metamorphic rock. Metamorphic rocks are formed by the transformation of pre-existing rocks due to heat, pressure, and chemical reactions. Therefore, the answer is the option that is NOT a metamorphic rock.
- Gabbro: This is an igneous rock that is formed from the solidification of molten magma. It is not a metamorphic rock.
- Therefore, the answer is gabbro.
In summary, the rock that is not metamorphic is gabbro.