Answer:
The correct answer is letter "B": buy both items until the marginal utility of a pair of jeans is three times the marginal utility of soccer balls.
Step-by-step explanation:
Marginal utility refers to the additional benefit or satisfaction gained from consuming one more unit of a good or service. To maximize marginal utility, every amount of money spent must represent the same amount of satisfaction for the consumer according to the value that good or service has.
In that sense, to maximize the marginal utility on Billie's purchase, the satisfaction gotten from the pair of jeans must be the same number of times as its value, and the satisfaction gotten from the soccer balls must be the same time as their value represent.