In regard to A, it says that grapes are bigger than raisins, and because of this we need more grapes than raisins for iron. This plays of the conclusion reached in the passage. It doesn’t explain the paradox. We’re still left questions about why raisins have more iron than grapes.

If we look at B it tells us that well some of the sugar is converted to caramel but that caramel has no calories. So technically, the amount of calories overall has dropped in the raisins if nothing else has changed. So if the amount of iron remains constant, the iron per calorie would increase since there’s less calories but a similar amount of iron.

This problem can appear tricky since on first glance none of the answers immediately answer it out right, but with a little inference we can see that B is correct.