The right answer to this question is: Newton's first law, the Law of Motion.
Explanation:
This law describes why the planets are pulled toward the sun in a very simple way. Every body in motion will continue like that, until another one interfers this process.
But in this process, we need to remember that the sun is very big, and bigger the body, bigger the gravity to pull the bodies around it towards it. The sun is very big, and this is how the planets keep moving around it.
