Choice "B" is correct. It would be negligent (i.e., a failure to exercise due care) to not correct discovered errors.
Choice "a" is incorrect. An auditor does not have a duty to discover all of a client's fraudulent activities.
Choice "c" is incorrect. A CPA is negligent if the CPA fails to use due care. Attempting to include a contract provision disclaiming liability for negligence is not itself a breach of care. In any case, the inclusion of such a disclaimer would probably be ineffective because it in essence would be an attempt to avoid responsibility to perform the audit with due care.
Choice "d" is incorrect. A CPA generally does not owe a duty to a client's customers. Indeed, reporting to the clients could violate a duty of confidentiality.