The correct answer is Mercury.Here we have changed the option in this question as in the official paper mars planet is also there which creates confusion.Concept:Planets:A celestial body moving in an elliptical orbit around a star is known as a planet.The planet of our solar system is divisible into two groups:Inner planet or terrestrial planets: they are made up of rocks and metals and relatively have high density. These are basically from Sun to the asteroid belt.Outer planets or Jovian planets: Mostly constituting gas.Inner or terrestrial planetsOuter plant or Jovian planetsMercuryJupiterVenusSaturnMarsUranusEarthNeptuneExplanation:Planets according to the radius of the Planet:Mercury: 2,440 kmMars: 3,390 kmVenus: 6,052 kmEarth: 6,371 kmNeptune: 24,622 kmUranus: 25,362 kmSaturn: 58,232 kmJupiter: 69,911 kmHence Mercury is smaller than Earth.Additional InformationPlanets order in Solar System: My Very Excited Mother Just Served Us NachosM ? Mercury, V ? Venus, E ? Earth, M ? Mars, J ? Jupiter, S ? Saturn, U ? Uranus, N ? Neptune