Concept:The planets in our solar system are arranged in order of their distance from the Sun. The closest planet is the one with the smallest average orbital radius.Explanation:Mercury is the innermost planet, orbiting the Sun at an average distance of about 58 million kilometres. It completes one orbit in about 88 Earth days, making it the fastest planet. Because it is so close to the Sun, its surface temperature swings dramatically between extreme heat and cold. Mercury has no natural satellites (moons). All other options – Saturn, Uranus, and Jupiter – are much farther from the Sun, lying beyond the asteroid belt and being gas giants with many moons.Answer:Mercury (Option B).