Answer:
b. the marginal benefit of the sixth banana exceeds its price.
Explanation:
Consumer consumes a commodity only until: that commodity consumption yields him/ her more or at least equal satisfaction than the - dissatisfaction from loosing money (price) spent at that commodity.
Marginal Benefit is in terms of Marginal Utility i.e Additional Satisfaction that consumer gains from consuming an additional unit of a commodity.
So, Christine will purchase 6th Banana only if additional satisfaction from that additional banana's consumption > dissatisfaction owing to price cost paid for it.
If Marginal Value < Price, its satisfactory loss making for her & she will rather reduce banana consumption. Average & Total Values are not apt tools to analyse the case.
