During 4 years on I don't think I've ever seen a question WORSE than this one ... at least not in the <Physics> department. This question is bad ! It's written from common misconceptions, it's misleading, and it can harm the learning of a student who tries to think about it too much, because it can teach him things that are false, and it'll take him a long time to UNlearn them. The person who wrote this question is himself unclear on the concept, and he should be required to re-qualify in Physics before he's allowed to put anything else out there for students to be exposed to.
Do you remember Newton ? Especially his Third Law of Motion, that says "For every action, there is an equal and opposite REaction." ? The all-time BEST example of this is: The force of gravity ALWAYS works in PAIRS. The gravity between two objects is always TWO forces. They pull in opposite directions, and they're EQUAL. The sad part is: When we're learning about gravity, we ALWAYS talk about one of them, and we NEVER talk about the other one. So everybody forgets that there's ALWAYS another one, pulling the other way.
-- The STRENGTH of gravity depends on the PRODUCT of the two masses. That means (mass-#1) TIMES (mass-#2). It doesn't matter if the objects are the same size and mass, or one of them is a planet and the other one is a mosquito. What matters is the product of their masses.
-- Any two objects pull together because of gravity. Gravity is always a PAIR of forces ... one in EACH direction. One force acts on object-A, pulling it toward object-B. The other force acts on object-B, and pulls it toward object-A.
===> The two forces are equal !
The forces of gravity between you and the Earth pull you and the Earth together. The strength of the forces depends on the product of (your mass) times (the Earth's mass). The strength of the force of gravity is what we call "your weight".
The two forces are equal ! YOUR weight on the Earth is EQUAL to the Earth's weight on YOU !
Now, let's look at this question again, together:
-- Why are objects pulled toward Earth's center ?
Because there is force ... we call it "gravity" ... between every two specks of mass in the Universe. In fact, there are always TWO of them, pulling in opposite directions. One force pulls the object toward Earth's center, and the other force pulls Earth toward the object's center. The two forces have equal strength.
The reason ?
A). Earth has a magnetic force that is strongest at its core. Yes, Earth has a magnetic field. It's NOT strongest at its core, and it has nothing to do with gravity anyway. Consider: The Moon has no magnetic field, but it has plenty of gravity. And there's a pair of equal, opposite gravitational forces between the Earth and the Moon.
B). Earth has a much greater mass than objects on its surface. Yes it does. But so what ? The strength of the gravity force depends on the product of both masses, not on which one is big or little. And the forces of gravity are equal in both directions anyway.
C). The weight of the atmosphere pushes objects toward Earth's surface. Yes it does. That's why we call it the "weight" of the atmosphere. The weight of a sack of rocks does the same thing. That's not the reason for the force that attracts you to the center of Earth, and attracts the Earth toward the center of you. Consider the Moon again. It has no atmosphere, but plenty of gravity. The atmosphere is not a reason for gravity.
D). The strength of the Sun’s gravity pushes down on objects at Earth's surface. The strength of the sun's gravity pulls things toward the Sun, not toward the Earth. And anything that gets pulled toward the Sun by gravity, pulls the Sun toward IT with an equal force. The reason that you're attracted toward the center of the Earth, and the Earth toward the center of you, has nothing to do with the Sun.
What you have to remember as you learn about gravity is:
-- Gravity between a pair of objects is always TWO forces. One force acts on each object, pulling it toward the other one.
-- Their strength depends on the product of both masses.
-- The two forces have EQUAL strength.
