Pesky little details like this can trip anyone up. In conversation, it's easily overlooked, but in writing, it stands out more. If you're committing the word to writing, make sure it's the right one!
The correct answer is "b) I am deciding among these three candidates."
Why? Excellent question! "Between" is only used for two items. Always use "among" for more than two. "Between you and me, ..." or "Among all of us, ..."
What about "amongst"? It's just a fun (okay, "old") version of "among": "Amongst all of us..." I love to use it, but then, I'm a grammar geek!
That lesson earned a snack! You can choose between the two treats left in the break room, or among all the goodies in the vending machine.