Only 24 humans have witnessed which celestial event?

A Syzygy
B Solar flares
C Earthrise

Just two dozen people have seen firsthand the view of Earth rising over the lunar horizon, and they were all members of Apollo missions that flew between December 1968 and December 1972. In fact, they are the only humans to ever leave low-Earth orbit or to see the entirety of the Earth in a single glance. Each of them reported being changed deeply by the experience.

The correct answer is Earthrise.