Final answer:
Neptune was discovered through the combined calculations of John Couch Adams and Urbain J. J. Le Verrier, leading to its observational discovery by Johann Galle and Heinrich d’Arrest; the correct answer from the options listed is 'e. None of these'.
Step-by-step explanation:
The individual responsible for discovering Neptune was not a single person but a result of the combined efforts of two mathematicians: John Couch Adams and Urbain J. J. Le Verrier. These two researchers independently calculated the position of Neptune using gravitational theory, following the observations of the 'disobedient' motion of Uranus. Their work led to the observational discovery of Neptune by Johann Gottfried Galle and Heinrich Louis d’Arrest on September 23, 1846, at the Berlin Observatory.
Therefore, the correct answer to who discovered Neptune among the provided options is 'e. None of these', as neither Clyde Tombaugh, Jan Hendrick Oort, Christian Huygens, nor Jean-Dominique Cassini can be credited for the discovery of Neptune which was a combined effort of Adams and Le Verrier.