Answer: 1. a. As a person consumes more and more of a certain good, the addition to total satisfaction decrease.
2. D.Quincy's marginal utility for ice cream increases and his marginal utility for pretzels decreases.
Explanation:
Diminishing marginal utility implies that one gains extra satisfaction from the consumption of an extra unit of a particular good but that as one continue to consume, there'll be dissatisfaction because the extra unit consumed will not bring more satisfaction like the former ones.
For example, if one is thirsty and buys water, an extra unit will at first give satisfaction but more extra water will not be as satisfying like the first one.
If Quincy eats less ice cream and more pritzels, the marginal utility for ice cream will increase as she'll get more satisfaction because she only consumed less while for pritzels, the satisfaction will reduce if she takes an extra.