Answer: B. Gravity
Step-by-step explanation:
Newton proposed that the force that kept planets orbiting around the sun in elliptical orbits was gravity. Much like the moon is pulled by earth's gravity, Newton proposed that the planets orbit the sun because they are being pulled in by the sun's gravity.
Being closer to the sun means that the gravity is stronger. It is for this reason that planets closer to the sun go around faster than than those further off and complete their orbits faster.