Important to notice is that requiring a password when waking up does not mean users must have passwords; it's a bit misleading named feature. It simply means that when waking up, Windows does not automatically log in the last user.
To achieve what you want, you must enable that setting, regardless if your user C has a password or not.
Here are the different wake up scenarios when the above mentioned setting is enabled and no user has automatic logon
1. Sleep initiated from a logged in user account > computer wakes up to that user's lock screen. (If the wake up password is not enabled and this user has no password or automatic logon is enabled > computer wakes up to this user's desktop):
2. Sleep initiated from logon screen when one or more users are logged in > computer wakes up to logon screen:
3. Sleep initiated from logon screen, no users logged in > computer wakes up to logon screen:
That's about it.