The correct answer is D) show specifically how the receiver or the receiver's company caused the problem.
Because he expects resistance, what he should do is to show specifically how the receiver or the receiver's company caused the problem.
As it is the second time he is going to expose his problem, he should write a very persuasive claim based on specific facts, more than in opinions. He has to mention in detail the way he was promised a rebate, by whom, when, and the reasons for that promise. He has to write using pathos to appeal to the emotion of the employee, making it very clear that he will go to a public office that supports consumers in the case he will have another "no" for an answer.