Question: According to NASA, what is the largest living organism visible from Earth orbit? Answer: Australia's Great Barrier Reef
"The Great Barrier Reef extends for 2,000 kilometers (1,240 miles) along the northeastern coast of Australia. It is not a single reef, but a vast maze of reefs, passages and coral cays (islands that are part of the reef). The white calcium carbonate that coats the living coral reflects light, making the water above the reef appear bright blue from space. This phenomenon allows the reef to be the largest living organism visible in National Aeronautics and Space Administration (NASA) satellite photos and videos ..."
Keith's note: Wrong. Coral reefs are collections of lots of organisms of many different species - not a single organism. "The largest living organism visible from Earth orbit" is most likely Pando - although larger examples may be awaiting discovery.
Pando (tree), Wikipedia
"Pando (Latin for "I spread"), also known as The Trembling Giant, is a clonal colony of a single male quaking aspen (Populus tremuloides) determined to be a single living organism by identical genetic markers and one massive underground root system. The plant is estimated to weigh collectively 6,000,000 kg (6,600 short tons), making it the heaviest known organism. The root system of Pando, at an estimated 80,000 years old, is among the oldest known living organisms."