What are your options if a Charmed Pendant reveals a Reaper King?

You will add WUBRG - i.e., one mana of each color - to your mana pool.

I'm not sure I'm answering your question, however, as you don't really have options. Perhaps you were wondering if you could choose to add W or 2, U or 2, etc? If so, then this ruling should explain why that doesn't happen:
11/1/2005: If you flip over a hybrid card, you get one of either color mana, not both, for each mana symbol.

Well I was thinking about this rule and if/how it effects this case:

106.8. If an effect would add mana represented by a hybrid mana symbol to a player’s mana pool, that
player chooses one half of that symbol. If a colored half is chosen, one mana of that color is added
to that player’s mana pool. If a colorless half is chosen, an amount of colorless mana represented by
that half’s number is added to that player’s mana pool.

But you don't add {2/w} etc. to your mana pool. Here's what you do: “For each colored mana symbol in that card's mana cost, add one mana of that color to your mana pool.” {2/w} is a colored mana symbol, and its color is white. So for this mana symbol you add one white mana = {w} to your mana pool. Proceed accordingly for the other mana symbols.

As Scott wrote, there are no options.

To be fair, the rules are a bit unclear on this specific interaction - Toby's question is definitely valid, esp. in light of 106.8.

However, Mario's explanation is as good as any - the Pendant is asking “what are the colored mana symbols”, not “what colors of mana do you want”.

And, I'll also point out that, given the removal of mana burn - it's largely irrelevant. Go ahead and add WUBRG, and be happy you don't lose a life for the U that no self-respecting mage would ever need to spend! :)