During twilight on 28 December around 1800 GMT, in DR position LAT 4°00'N, LONG 0°06'W, the sextant altitude (hs) of Venus was 30°46.8'. The height of eye was 36 feet, and the index error was 2.0' on the arc. The temperature was 68°F. The barometer read 1030 mb. Calculate the observed altitude (Ho).

Correct Answer: Option D — Ho 30°38.1'

Correct the sextant altitude by applying index error (on = add), dip for 36 feet, and refraction. Apply the additional corrections for Venus, temperature, and pressure as specified in the Nautical Almanac. The final observed altitude (Ho) is 30°38.1', calculated according to standard celestial navigation reduction principles.