Answer: (D) buy more ice cream and fewer cookies
Explanation: When a consumer spends all of his income and consumes a bundle of goods such that the marginal utility per dollar from all goods is equal, then the consumer's total utility is maximized. Buying more of ice-cream gives the only combination that gives him the same marginal utility per dollar Bobby spends on cookies as on ice cream.
Marginal utility estimates the added satisfaction a consumer gets from consuming additional units of goods or services. It is used to determine how much of a product consumers are willing to purchase.