Choice "A" is correct. If the principal is undisclosed, the third party with whom the agent dealt can hold the agent liable on the contract.
Choice "c" is incorrect. A third party has no general right to discover the identity of an undisclosed principal.
Choice "b" is incorrect. A third party never has the right to force a principal to ratify a contract; only a principal has a right to choose to ratify an unauthorized contract.
Choice "d" is incorrect. As a general rule a third party who has contracted with an agent for an undisclosed principal has no right to rescind the contract upon subsequent disclosure of the principal. Such a right exists only if the nondisclosure was fraudulent.