Which of the Following Sentences Will Have the Most Positive

Which of the following sentences will have the most positive effect?

A) We cannot provide a refund for the purchased product.
B) Your inability to produce the purchase invoice has resulted in a refund being refused.
C) We can offer you a refund if you can provide us with the purchase invoice.
D) While a refund is not possible at the given time, we can do so if you submit the purchase invoice.
E) We cannot provide a refund unless you provide the purchase invoice.