The correct answer is option 4, Mercury. There are four planets listed in the options: Neptune, Jupiter, Mars, and Mercury. The question asks for the planet with the least mass. To determine the planet with the least mass, we need to compare the masses of each planet.
Mercury is the smallest planet and also has the least mass. It is approximately 1/18 the mass of Earth, making it the lightest planet in our solar system.
Neptune is the fourth-largest planet and has a greater mass compared to the other options.
Jupiter is the largest planet and has the most mass among all the planets in our solar system.
Mars is the second smallest planet and has more mass than Mercury, but less mass compared to Neptune and Jupiter.
Therefore, Mercury is the correct answer as it has the least mass among the four listed planets.