The correct answer is B) Altitude. In the horizon system of coordinates, altitude is equivalent to the declination of the equatorial system. Altitude is the angular distance of a celestial body above or below the celestial horizon, measured vertically. This is directly analogous to declination, which is the angular distance of a celestial body north or south of the celestial equator. The other options are incorrect because they do not directly correspond to declination in the equatorial system. Nadir is the point directly opposite the zenith, azimuth is the horizontal angle of a celestial body, and zenith distance is the angular distance of a body from the zenith.