C. The speed is slowest when it is farthest away from the Sun
Step-by-step explanation:
This fact is well explained by Kepler's second law of planetary motion, which states that:
"A line joining a planet and the Sun sweeps out equal areas during equal intervals of time"
This means that this line connecting the planet and the Sun must go slower when the planet is farther from the Sun (because the line is larger, so it can cover more area in the same time). So, the speed of the planet is slower when the planet is farther from the Sun.