Final answer:
Without a specific context or book reference, the exact instance of Snape mocking Harry about ghosts cannot be provided. Snape's ridicule is part of their ongoing tense interactions in the Harry Potter series.
Step-by-step explanation:
The scene you are asking about takes place in the Harry Potter series, written by J.K. Rowling. Unfortunately, without a specific reference to which book and the context of this exchange with Professor Snape, it is challenging to provide the exact moment you're referring to.
In the Harry Potter series, Snape often mocks Harry for many things. One moment that stands out is when Snape ridicules Harry's response about the nature of ghosts during one of his Potions classes in The Sorcerer's Stone. This condescending treatment is characteristic of the contentious relationship Snape maintains with Harry throughout the series. For a more precise answer, please provide the book or specific situation you are referring to.