You're looking for the answer of 'The Great Wall of China,' but that is not correct! Any number of man-made structures can be seen from space, provided we construe 'structure' to mean 'anything built.' Many of these are things that look like long, straight lines when seen from afar, such as highways, railroads, canals, and of course walls. If the orbit is low enough you can see even more. There is a photo of Cape Canaveral taken during the Gemini V flight in which the big Launch Complex 39, used for the Apollo missions, is clearly visible. Another photo of the Nile delta, taken from a height of 100 miles, shows an extensive road network. Gemini V astronauts Gordon Cooper and Charles Conrad were able to spot, among other things, a special checkerboard pattern that had been laid out in Texas, a rocket-sled test in New Mexico, and the aircraft carrier that would later pick them up in the Atlantic, along with a destroyer trailing in its wake.
If question is asked "...only man made structure able to be seen from the moon" - still NOT the Great Wall. According to NASA, the earth as seen from the moon takes up less than one degree of arc in the sky. Basically it looks like a big blue marble. No man-made detail can be seen at all; sometimes even the continents are barely distinguishable. The NASA folks, I gather, are getting a little tired of hearing about the Great Wall of China. Nobody knows exactly where the story got started, although some think it was speculation by some bigshot during an after-dinner speech in the early days of the space program. The Teeming Millions are humbly requested to give it a rest.Tom Burnam, author of More Misinformation (1980), quotes a letter from astronaut Alan Bean on the subject: "The only thing you can see from the moon is a beautiful sphere, mostly white (clouds), some blue (ocean), patches of yellow (deserts), and every once in a while some green vegetation. No man-made object is visible on this scale. In fact, when first leaving earth's orbit and only a few thousand miles away, no man-made object is visible at that point either."