The expression "first magnitude" is usually used to refer only to bodies of magnitude __________.

Correct Answer: Option D — 1.5 and greater

In celestial navigation, the term 'first magnitude' is a classification for the brightest stars. According to standard astronomical conventions used in the Nautical Almanac, this category includes stars with a magnitude of 1.5 or brighter, providing the navigator with the most reliable and easily identifiable bodies for celestial fixes.