B is incorrect because Mary doesn't have to account for the possibility that her employees have rights related to the sale. From the stimulus, we understand that Mary can sell whenever she chooses. Perhaps the rights of her employees are related to payment as opposed to decision making in the selling process.
Her conclusion is that Jamal has made two incompatible statements. But that would be true if both of his statements were referring to the same idea.
In the first statement, he refers to legal right, whereas in the second statement, he uses "right" in the moral sense.
She doesn't catch this distinction and pointing it out using answer choice D, weakens her argument.