What best explains why planets move across the stars within a narrow band known as the zodiac?

The movement of planets across the stars within the zodiac can be best explained by the orbits of the planets lying in approximately the same plane. This plane, known as the ecliptic plane, is aligned with the Earth's orbital plane around the Sun. Since the planets in our solar system generally orbit the Sun in this same plane, their paths, as observed from Earth, will appear to be confined to this narrow band in the sky, which is the zodiac.

This alignment is largely due to the way the solar system formed from a rotating disk of gas and dust, leading to most celestial bodies sharing a common orbital plane. Consequently, as planets travel around the Sun, they remain in this ecliptic plane, resulting in their visibility along the zodiac as seen from Earth. Thus, option B accurately reflects the fundamental reason for the observed motion of planets within the zodiac.