This answer will vary a lot, depending on who you will ask. If you're asking proponents of the LCHF (low carbs, high fat) diet, they would say you should reduce your intake of carbohydrates down to around 10 to 20 percent, making either A or C your desired answer.
If you would ask people who propose the current nutritional guidelines as the best way of eating, the amount of caloric intake that should come from carbs according to them would be more in the range of around 40%, making B the correct answer.
Scientific theories can never be proven true beyond all doubt; they can only be supported by a wide body of evidence. Only one of the statements that follow uses the term theory in its correct, scientific sense.
