In the following question select the one which is different from the other three responses:

Sun

The option that is different from the other three responses is:

☀ Sun

 "Saturn," "Mercury," and "Pluto" are all planets in our solar system, but the "Sun" is not a planet. The Sun is a star, specifically the star at the center of our solar system, around which all the planets, including Saturn, Mercury, and Pluto, revolve. Therefore, "Sun" is the different option in the given list.