Titan is the natural satellite of which planet?

A natural satellite (moon) is a celestial body that orbits a planet under that planet's gravity, and each major moon in the Solar System is uniquely and permanently paired with the specific planet it orbits; identifying "which planet a named moon belongs to" is a matter of matching the moon to its well-established host body.

Titan is the largest moon of Saturn -- the second-largest moon in the entire Solar System after Jupiter's Ganymede -- and was discovered by the Dutch astronomer Christiaan Huygens in 1655. It is Saturn's most massive satellite and the only moon in the Solar System known to sustain a dense, nitrogen-rich atmosphere, which is why Titan is decisively identified with Saturn.

• Neptune's largest and best-known moon is Triton, not Titan, so Neptune is not the host planet.

• Mercury has no natural satellites at all, so it cannot be paired with any moon, including Titan.

• Venus likewise has no natural satellites, which rules it out in the same way as Mercury.

Since Titan's only correct planetary pairing among the given options is Saturn, the answer is Saturn.