The correct answer is D) Altitude. The celestial navigational triangle consists of three sides: zenith distance, co-latitude, and co-declination. Altitude is not a side of the celestial navigational triangle, but rather a measurement derived from the other three sides. Altitude is the angular distance of a celestial body above the horizon, and it is calculated using the other three elements of the celestial navigational triangle. The other answer choices, A) Zenith distance, B) Co-latitude, and C) Co-declination, are all sides of the celestial navigational triangle and are essential elements in the calculation of a vessel's position using celestial navigation.