Final answer:
The speed of a planet's revolution is primarily affected by the Gravity of the sun, which governs the motion of the planets in the solar system.
Step-by-step explanation:
The speed of a planet's revolution around the sun is mainly affected by gravity. The answer to the question is B) Gravity of the sun. The sun's gravity is the dominant force that governs the motion of planets in our solar system, and the speed of revolution varies with the distance from the sun due to this gravitational force. According to Kepler's laws of planetary motion, a planet moves faster when it is closer to the sun and slower when it is farther away. This phenomena is elegantly described by Kepler's second law, which states that the line connecting a planet to the sun sweeps out equal areas during equal intervals of time, resulting in varying speeds along the orbit.
Although other factors such as gravitational interactions with other planets and the shape of a planet's orbit can cause slight variations, none of the other options provided (A) Shape of the planet, (C) Temperature of the sun, or (D) Atmosphere of the planet directly affect a planet's revolution speed in the same manner that the sun's gravity does. Instead, these factors may influence other aspects of a planet's characteristics and behavior.