The correct answer is D) Sixth. Under ideal viewing conditions, the dimmest star that can be seen with the unaided eye is of the sixth magnitude. This is based on the Bortle scale, which is a nine-level numeric scale that measures the night sky's brightness of a particular location. The sixth magnitude represents the dimmest stars visible to the naked eye in a clear, dark sky away from light pollution. The other answer choices are incorrect because the first magnitude represents the brightest stars, the third magnitude represents moderately bright stars, and the fourth magnitude represents stars of average brightness, all of which are brighter than the sixth magnitude stars that represent the dimmest visible to the unaided eye.