Option-1: Mars is a cold planet, but it is not the coldest compared to the other options.
Option-2: Earth has varying temperatures, but overall it is not the coldest compared to the other options.
Option-3: Pluto is the correct answer. It is the coldest among the given options. Pluto is a dwarf planet located farthest from the sun, which results in extremely low temperatures.
Option-4: Mercury is the closest planet to the sun and has temperatures that can exceed 800 degrees Fahrenheit, making it much hotter than the other options.
Therefore, option-3, Pluto, is the coldest among the given options.