The correct answer is - the atmosphere.
All of the terrestrial planets have atmospheres, but all them differ in their composition and density too. Venus has the highest temperature of the terrestrial planets, higher even then Mercury despite being closer to the Sun, because it has very dense atmosphere. Also, the key moment with the dense atmosphere of Venus is that it is mostly composed of carbon dioxide. The carbon dioxide is a gas that has the ability to trap the solar radiation into the atmosphere, so the more there is from this gas, the hotter it will be, and Venus has exactly that.