Answer: Buy more of both rice and beans
Explanation: Marginal benefit refers to the additional benefit that a customer get by consuming one additional unit of a commodity.
In the given case, the marginal benefit for the customer is positive for both of the goods. Also if he chooses to but one more unit of anything in place of other than he will not able to use his budget properly.
Thus, from the above we can conclude that the correct option is C.