Final answer:
The gravitational pull of the Sun is the factor that keeps Earth in its orbit, as it provides the centripetal force needed to maintain the elliptical path per Newton's law of universal gravitation.
Step-by-step explanation:
The factor that keeps Earth in orbit around the Sun is B) The gravitational pull of the Sun. According to Newton's law of universal gravitation, every mass exerts an attractive force on every other mass. This law explains that the force of gravity is what keeps planets, including Earth, in orbit around the Sun. Gravity provides the necessary centripetal force to keep Earth in its elliptical path around the Sun. The gravity gradient, which is the difference in the pull of gravity over distance, is a concept applicable to the orbits of artificial satellites like the elongated bell-shaped ones, and explains why the Moon, despite being pulled by Earth's gravitational force, can have a stable orbit.