"the gravity of the sun causes the planets to move in a circular path."
which law of motion is this?
a) first law
b) second law
c) third law

You're thinking of Kepler's first law of planetary motion, but that's not
what it says.

First of all, Kepler didn't say anything about gravity.  He only described
the orbits.  And he said that the orbit of each planet is an ellipse, with the
sun at one focus.

If by some chance you're thinking of Newton ... he showed that if his formula for
gravity is correct, then the orbit of a planet must be an ellipse.  But Newton's law
of gravity is not one of his so-called "three laws of motion".