Short answer: using the form of answer C, the correct grammar would be: "No, there isn't."
The presence of the term "is there" in the question means it is asking whether a discount exists. The correct response should use the same "is there" form to affirm or reject the existence, or perhaps a similar phrase. "No, none whatsoever." is such a similar phrase.
The phrase: "No, it isn't." is used to say the object (it) is not something. For example:
Is the discount available today?
No, it isn't.
The response is actually short for "No it isn't available today." Because the question does not ask whether the discount is something, but asks whether it exists, the answer cannot merely be "No it isn't."