the chief among which was cost
The word 'chief' means main or principal, and all three words mean the top one. And when we refer to a member of a group, we use of. For example, if we want to name our best friend, we'd say He is the best of my friends, but not He is the best among my friends.
Now, run these through your head to see how they sound:
·The main reason of all the reasons.
·The main reason among all the reasons.
The first sounds better and is more precise.
So, the answer is option (b).