Which of these is not a moon of Saturn?

Adrastea, also known as Jupiter XV, is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in photographs taken by Voyager 2 in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through a telescope. It was officially named after the mythological Adrasteia, foster mother of the Greek god Zeus—the equivalent of the Roman god Jupiter.

Adrastea is one of the few moons in the Solar System known to orbit its planet in less than the length of that planet's day. It orbits at the edge of Jupiter's Main Ring and is thought to be the main contributor of material to the Rings of Jupiter. Despite observations made in the 1990s by the Galileo spacecraft, very little is known about the moon's physical characteristics other than its size and the fact that it is tidally locked to Jupiter.

Atlas is an inner satellite of Saturn. Atlas was discovered by Richard Terrile in 1980. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth.

Prometheus is an inner satellite of Saturn. It was discovered in 1980. In late 1985 it was officially named after Prometheus, a Titan in Greek mythology.

Hyperion, also known as Saturn VII (7), is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848.