Tom enjoys drinking Diet Coke with his lunch, but his enjoyment decreases after each glass he consumes. This is an illustration of

Answer: diminishing marginal utility

Explanation:

Diminishing Marginal Utility states that all else equal as consumption increases the marginal utility derived from each additional unit declines. In another word the satisfaction or utility that they derive from the product wanes as they consume more and more of that product. The law of diminishing marginal utility is one of the vital laws of economics