Answer:
The revision which uses a formal tone is the following one:
C. An interesting fact about Greek mythology is its connection to many modern names. One example is East Sparta, Ohio, a town that takes its name from the home of Greece’s famous warriors.
Step-by-step explanation:
If you want to achieve a formal tone, you cannot sound as if you were simply having a casual conversation, asking questions like: "Isn’t it interesting how...?", or "Have you noticed how...?". Plus, you cannot express yourself in first person singular, that is, by using the pronoun "I", as in: "One interesting fact I found...", and "I didn’t know until now that...". If you want to sound formal, you must be neutral about what you talk or write about, which means you should not use a familiarity tone.
All of that being said, letter C is the way to go, since all that should be avoided at all costs when seeking a formal tone is present in the other alternatives. In fact, letter C is an example of written formality: it sounds serious, third person singular is used the whole time and there is no display of familiarity or overexcitement whatsoever.