D. The amount spent on grapes compared with the weight of the purchase. In the other examples, the rate of change would be either very erratic (people on the bus, which would change suddenly and in large amounts depending upon stops, the girl's shoe size, which would change somewhat consistently, but with swings as growth spurts happened, or not at all, like the pizza example, which if you think about it, the amount of pizza doesn't really matter). The grape answer would have a set amount of pay based on weight (say $1 per pound), which would have a constant rate of change tied to the weight.
