Final answer:
Correct option a.) a combination of random and orderly motions. Stars in the Milky Way disk exhibit a combination of random and orderly motion, primarily moving in nearly circular orbits that are orderly within the galactic plane, affected by dark matter and obeying Kepler's third law.
Step-by-step explanation:
The statement that best describes the orbital motion of stars in the disk of the Milky Way is 'a combination of random and orderly motions'. The disk stars are indeed in motion, and they exhibit primarily orderly motion within the galactic plane, following nearly circular orbits. The randomness comes from variations in velocities and slight inclinations to the galactic plane, but the aggregate motion contributes to the rotational symmetry of the Galactic disk. The motion of stars in the Galaxy's halo, on the other hand, is more randomly oriented and their orbits are highly elliptical. This is in contrast to the disk stars which follow the galactic rotation in a more systematic pattern, which is influenced by the gravitational effects of the Galaxy's mass distribution, including the mysterious dark matter that adds to the gravitational pull without emitting light.
Stars in the disk of the Milky Way also obey Kepler's third law, meaning objects farther from the center take longer to orbit the Galaxy than do those closer to the center, a phenomenon known as differential galactic rotation. This is similar to the solar system where the planets and other objects revolve around the Sun in the same direction, which is also mostly in the plane of the Sun's rotation.