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Option C: keep Ninetales pure Fire-type but allow it to learn some Magic-type moves.

Several reasons here...

1: The time to add new types to old Pokémon is in the first generation that the new type is added, not one or two generations later. Look at Magnemite/Magneton for example; they were given the Steel type in G/S/C when Steel was first introduced. No other old Pokémon have gotten new types in later generations.

2: Pokédex entries, much like the anime, should never, ever be the sole reason for adding a type, move, etc. to a Pokémon. They get pretty ridiculous at times, and some of them are just plain wrong--the "9 wizards" story, for example. A Ninetales comes from a Vulpix being exposed to the radiation of a Fire Stone, not from a bunch of wizards sneaking into your room while you're asleep, making your Vulpix and one of your Fire Stones disappear, and then fusing into a Ninetales.
Basically, if a Pokédex entry sounds like some kind of ridiculous, exaggerated, and completely untrue folktale... chances ere that's exactly what it is. If "Cat" had a Pokédex entry, I wouldn't be surprised at all to see some mention of how it sucks the breath from babies or how the ones with black fur are bad luck.

3: Nintendo/Gamefreak themselves have been choosing Option C all along! The only difference there is that the official series doesn't have a Magic type. Notice that with every new version, Vulpix/Ninetales seems to get more Ghost-type and Psychic-type moves, as well as moves with mystical or ghostly overtones like Safeguard (which was usually fan-translated as "Mystical Protection" back before Gold/Silver were officially translated into English) and Will-O'-Wisp?