Why C? And why not the others?

Correct answer: C. Ganymede

Ganymede, orbiting Jupiter, spans about 5,260 kilometres — making it larger than the planet Mercury. It is also the only moon known to generate its own magnetic field, and evidence points to a buried saltwater ocean holding more water than all of Earth's surface oceans. Saturn's Titan is the classic near-miss: it is the second largest and famous for its thick atmosphere, so it often steals the credit. Earth's Moon feels enormous in our sky but ranks only fifth, and Europa, though fascinating for its hidden ocean, is smaller than our Moon.

Source: NASA Science: Ganymede facts (science.nasa.gov/jupiter/jupiter-moons/ganymede/facts/)