The phenomenon known as precession is due to a regular, periodic variation in what property of Earth's motion

The answer is... "the direction in which its rotational axis points"

Explanation:

This regular, periodic wobble is introduced by the competing gravitational tugs between Earth, the Moon, and the Sun.

It is the direction of the tilt that changes, so the North Star (i.e., the star closest to the north celestial pole) changes in a regular, predictable way with time.

Hope this helps!! ;)

answer(c) carved on a babylonian clay tablet that showed earth as a flat circular disk;