3. (b) That's right. The problem for C in this case is that B has not told A that he is acting for C, as an agent. The existence of C as a principal has not been disclosed to A and so A would think that B is buying the stamps for himself.
An undisclosed principal is not entitled to ratify an unauthorised act. Ratification is only permitted when the third party is aware that the person they are dealing with is acting as an agent, and when the principal is either identified or identifiable as a particular person. Only the person identified or identifiable as the principal at the time of the transaction is then entitled to subsequently ratify.
A principal cannot, by ratification, give validity to any transaction for which the principal lacked capacity for, or which was for some reason legally void, either at the time that the transaction was entered into, or at the time of attempted ratification.