B is corrent because the doctrine of respondeat superior provides that an employer is responsible for the torts committed by employees in the normal scope of duties.
A is incorrect because this doctrine involves acceptance of the terms of an agreement.
C is incorrect because estoppel prevents a party to a contract from being harmed if there is an expectation that the other party to the contract was contracting consistent with his or her powers.
D is incorrect because ultra vires acts are acts that are beyond the scope of corporate powers.
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