Final answer:
The celestial equator is the correct answer, lying halfway between the celestial poles and serving as an essential reference for celestial navigation.
Step-by-step explanation:
The correct answer to the question is e. celestial equator. The celestial equator is an imaginary extension of Earth's equator onto the celestial sphere, and it lies halfway between the celestial poles. This great circle divides the sky into the northern and southern celestial hemispheres. If you were standing on the Earth's equator, you would see the celestial equator pass directly through your zenith. On Earth's poles, the celestial equator would align with the horizon but from different hemispherical vantage points. The celestial equator serves as a useful reference point for celestial navigation and observing the stars' apparent motions in the sky.