Final answer:
Mars is cold because it has a very thin atmosphere with little CO₂ to retain heat, it has lower gravity which allowed gases to escape, and it's farther from the Sun receiving less solar energy.
Step-by-step explanation:
Despite the fact that carbon dioxide (CO₂) is a powerful greenhouse gas, Mars remains very cold for a number of reasons. Firstly, Mars has a very thin atmosphere, which means it cannot retain as much heat as Earth or Venus. Moreover, Mars's atmosphere parallels that found extremely high in Earth's atmosphere where the air is thin and less capable of holding warmth. This thinness of the Martian atmosphere leads to only about a 2 °C worth of greenhouse warming. Secondly, Mars is a smaller planet with lower gravity which allowed atmospheric gases to escape into space, resulting in a cooling effect. Finally, Mars's greater distance from the Sun compared to Earth also means that it receives less solar energy to begin with, contributing to its colder climate.
Given these factors, the correct answer to the question would be that there is not enough CO₂ on Mars for the greenhouse effect to work effectively (e).