Concept:An indifference curve shows different combinations of two goods that give a consumer the same level of satisfaction.Explanation:Each point on an indifference curve represents a pair of quantities of two goods.The consumer is indifferent between these pairs because every point gives equal utility.Therefore, all pairs from which the consumer derives equal satisfaction are plotted on the same indifference curve.A budget line shows affordable combinations, while an isoquant measures production, so they are not correct here.Answer:A. An indifference curve