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

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".)

Wavelength divides in half and speed stays the same.