The answer is C.
The concept of declining marginal benefits means that for every additional (marginal) unit you buy or consume, the benefit you receive decreases (declines).
This is most clearly illustrated in answer choice C, where you get less and less benefit from each additional car you buy. By the third car, there's not even any benefit to buying another.
Answer choice D is tempting because the price declines with each additional unit, but this would be declining marginal *cost*, not benefit.