Answer: X

The human mind is a pattern seeking one.  We humans are quite good at turning abstract shapes into familiar ones.  One of the most famous astronomical examples of this is the Man in the Moon–from our terrestrial vantage point the craters on the Moon’s surface look like a man’s face looking down on us.

If you look even closer at the Moon, however, you will find a much tinier surface feature just waiting for us to visualize a symbol over it.  For a scarce four hours at the beginning of the first quarter, the horizon of the Moon is illuminated by the Sun in just such a way that the walls of the lunar craters La Caille, Banchinus, and Purbach cast shadows that create the illusion of a giant X on the surface of the Moon. This formation is known as the Purbach Cross (thanks to the related crater) or the Werner X (named so after the Werner crater, a sizable landmark just south of the X formation).

Bonus Trivia: Formations like the Purbach Cross are known as clair-obscur formations, or light-dark formations, created by extreme contrast between natural light (in this case the sun) and darkness (the deep darkness of the dark side of the Moon and the Moon’s craters).