Planet X
Explanation:
The orbital radii of planet X is the smallest which means that is closest to the sun. The distance to the star determines the temperature of the planet. Closer the planet, more likely it would have higher temperature. So planet X has highest temperature most likely.
In the solar system, there is exception, mercury is closest to sun but Venus which is the second planet from the sun has highest temperature. This is because mercury has no atmosphere and Venus has very thick atmosphere which traps the solar radiation and heats up.
