Option 1: Improve - This option is incorrect. According to the law of diminishing marginal utility, as the amount of a good consumed increases, the marginal utility of that good tends to diminish, not improve.

Option 2: Diminish - This option is correct. The law of diminishing marginal utility states that as an individual consumes more of a good, the additional satisfaction or utility derived from each additional unit of the good decreases. In other words, the more you have of something, the less satisfaction or utility you derive from consuming more of it.

Option 3: Remain constant - This option is incorrect. The law of diminishing marginal utility suggests that utility decreases as consumption increases, so the marginal utility of a good does not remain constant.

Option 4: First diminish and then improve - This option is incorrect. The law of diminishing marginal utility implies that the marginal utility will continuously diminish as more of a good is consumed. It does not then improve.

In conclusion, option 2 is the correct answer. The law of diminishing marginal utility states that as the amount of a good consumed increases, the marginal utility of that good tends to diminish.