Explanation
Why Correct: Mercury orbits the Sun at an average distance of about 57.9 million km, the smallest orbital radius of all planets in the Solar System. It is the closest planet to the Sun.
Distractor Analysis: Mars is the fourth planet from the Sun, at about 227.9 million km. Venus is the second planet, at about 108.2 million km. Saturn is the sixth planet, at about 1.43 billion km. All are farther than Mercury.
Takeaway: Planet order from Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.