Answer: The British Government
During World War II, British pilots began downing an unusual number of German aircraft and with uncanny precision at that. The secret of their success was the British development of radar systems, which made locating incoming German aircraft easy regardless of the time of day or atmospheric conditions. The existence of radar, however, was a closely guarded secret.
As such, when the press questioned how the British pilots were so successful, the British government and the Royal Air Force responded that it was all the carrots their pilots ate. While it’s true that carrots are packed with vitamins (like Vitamin A) that are critical for optimum eye health, eating plate loads of them won’t improve your vision or give you the ability to spot Nazi planes ten miles out.
The British did such an excellent job of keeping a straight face (and supporting their lie with propaganda posters and supporting documents) that the idea of carrots improving your vision (especially your night vision) remains firmly embedded in popular lore over half a century later.