The correct answer is D. Ganymede. Ganymede is the largest and most massive of the Solar Systems moons. The ninth-largest object (including the Sun) of the Solar System, it is the largest without a substanial atmosphere. It has a diameter of 5.268 km (3,273 mi), making it 26% larger than the planet Mercury by volume, although it is only 45% as massive. The moon has a metallic core. It has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field.