You really have to dig through the language of this one.
The conclusion is:
If an action being morally right means it has the best consequences, then we can never know for sure if an action is morally right (i.e., we can never know whether that action has the best consequences).
Why? Because we can never know all of an actions consequences... only some of them.
In other words, the author's suggesting that we can never know the best consequences of an action because we can't know ALL of the consequences. That's what (C) is suggesting -- that we'd have to know ALL of the consequences in order to know which ones are best.
I know -- it's ugly. I hope this helps.