Find the word that is different from the other three: Cloud, Vapour, Rain, Fog
Find the word that is different from the other three: Cloud, Vapour, Rain, Fog
Cloud, Rain and Fog are all visible forms of water in the atmosphere (droplets we can see). Vapour is water in its invisible gaseous state, so it does not belong with the other three visible phenomena.
Re-check your working by substituting the answer back into the original question.