If I answered your question with simply, "That is for you to decide", it would mean a) - that is, No one here is going to tell you; you have to work it out yourself.
If I gave you a better reply, and then said, "I think this is right. It's for you to decide whether to believe me", that is b). You can choose to believe me or not, or you can choose not to make that decision. It's up to you.
Can you think of different contexts for 3 which would make it have the two meanings?
- For Teachers