Question

Which law best describes why the planets are pulled toward the Sun and orbit it? Newton's law of universal gravitation Kepler's law of ellipses Newton's 2nd law of motion Newton's 3rd law of motion

Answer 1

The forces that attract the sun and the planet toward
each other are described almost exactly by Newton's
law of universal gravitation.

In order to derive the existence of orbits, you also need
Newton's second law of motion.

Once you have properly mashed and massaged these two
laws together on paper, you GET Kepler's law of ellipses,
plus Kepler's other two laws of planetary motion. 

It's really a beautiful math exercise and experience. 
The agreement is so perfect that it gives you, right there on
your paper, powerful powerful arguments for the correctness
and accuracy of both Kepler's laws AND Newton's laws.
(But not proofs.  They're all "only theories".)

Answer 2

Answer: Newton's law of universal gravitation

Explanation:

We know that an object does not change its state of rest or motion unless and until an external unbalanced force acts on it.  Planets would have continued to move in straight path if they would have not pulled towards the Sun due to gravity. Gravitational pull of the Sun is the reason why the planets are pulled towards the Sun and orbit it.

Hence, Newton's law of universal gravitation which states that gravitational force exists between any two bodies having mass is the correct answer.