Choice "B" is correct. Under the doctrine of respondeat superior, a principal, including a corporation, can be held liable for an employee's tort committed within the scope of employment.
Choice "d" is incorrect. Ultra vires is a doctrine limiting a corporation's power to act outside the scope of the corporation's stated purposes or statutory powers.
Choice "a" is incorrect. Corporation by estoppel prevents (stops) a party to a lawsuit from successfully arguing that an entity is not a corporation on account of the failure of the entity to comply fully and timely with all the provisions of state law with respect to the formation and operation of a corporation. The party is so prevented ("estopped") on account of that party's previously treating the entity as if the entity had been a properly-formed and properly-operated corporation.Choice "c" is incorrect. Ratification occurs when a principal agrees to be bound by a previously unauthorized contract entered into by an agent on the principal's behalf.