Final answer:
The synonym for "whim" is "fancy," which like a whim, denotes an impulsive thought or desire. "Notion" and "humor" are not as closely related, and "horror" is an antonym.
Step-by-step explanation:
The synonym for "whim" as it is used in the question is fancy. This word implies a sudden thought or desire that is not based on reason, aligning with the definition of whim. Notion and humor can be similar in meaning, but they are not as closely related to the impulsive nature of whim, which can also be associated with an epiphany or a sudden intuitive understanding. While a whim often arises spontaneously, it differs from a philosophical or mathematical intuition which is more about clear and distinct understanding. Horror, provided as an option, is an antonym to whim and therefore is not the synonym we're searching for.