The correct option is D. decreases as additional units are consumed. Explanation of the Correct Answer Marginal Utility refers to the additional satisfaction or benefit that a consumer derives from consuming one more unit of a good or service. The concept is rooted in the principle of diminishing marginal utility, which states that as a person consumes more units of a good, the additional satisfaction gained from each subsequent unit tends to decrease.

1. Understanding Diminishing Marginal Utility:
2. When you first consume a good, such as a slice of pizza, the satisfaction (utility) you get from that first slice is usually quite high because you are hungry.
3. As you continue to eat more slices, the additional satisfaction you receive from each new slice tends to decrease. By the time you reach the third or fourth slice, you may still enjoy it, but the extra satisfaction you get from each additional slice is less than the satisfaction from the first one.
4. Graphical Representation:
5. If you were to graph marginal utility against the quantity consumed, you would see a downward-sloping curve. The first few units may provide high marginal utility, but as consumption increases, the curve flattens out, indicating that the marginal utility is decreasing.
6. Real-World Examples:
7. Think about drinking water. The first glass quenches your thirst significantly, providing high utility. The second glass still provides satisfaction, but not as much as the first. By the time you reach the fifth or sixth glass, the additional satisfaction is minimal, and you may even feel uncomfortable.

Why the Other Options Are Incorrect A. Increases as additional units are consumed: - This option suggests that the satisfaction from each additional unit increases, which contradicts the principle of diminishing marginal utility. In reality, as you consume more, the additional satisfaction typically decreases, not increases. B. Increases at a decreasing rate as additional units are consumed: - This option implies that while marginal utility may increase, it does so at a decreasing rate. This is also incorrect because it suggests that marginal utility can still be increasing, which is not the case. The correct understanding is that marginal utility decreases as more units are consumed. C. Decreases at a constant rate as additional units are consumed: - This option suggests that the decrease in marginal utility happens at a constant rate, which is not typically how marginal utility behaves. The decrease is usually not linear; it tends to slow down as consumption increases, meaning the rate of decrease itself is not constant. Common Pitfalls

• Confusing Total Utility with Marginal Utility: Total utility can increase even as marginal utility decreases. It's important to distinguish between the two concepts.
• Assuming All Goods Follow the Same Pattern: While diminishing marginal utility is a common phenomenon, some goods (like addictive substances) may not follow this pattern.
• Ignoring Individual Preferences: Different individuals may experience varying levels of satisfaction from the same good, which can affect their marginal utility.

Revision Summary

• Marginal utility decreases as more units of a good are consumed due to the principle of diminishing marginal utility.
• The first units of consumption provide the highest satisfaction, which diminishes with each additional unit.
• Graphically, marginal utility typically shows a downward-sloping curve.
• Understanding the difference between total utility and marginal utility is crucial for grasping consumer behavior in economics.