The correct answer is D. Polar distance.
Polar distance is the angular distance between a celestial object and the north or south celestial pole. It is measured along the celestial equator, from the object to the pole.
Co-declination is the angular distance between a celestial object and the celestial equator. It is measured along the meridian of the observer, from the object to the equator.
Co-altitude is the angular distance between a celestial object and the horizon. It is measured along the vertical circle of the observer, from the object to the horizon.
Co-latitude is the angular distance between the observer’s latitude and the equator. It is measured along the meridian of the observer, from the observer’s latitude to the equator.