Answer
Option d.
Work Step by Step
Marginal utility is defined as the "extra satisfaction from the consumption of one more unit of some good or service."
Let's try to understand it clearly with an example. Assume that there is some method to measure the level of satisfaction you get when you do something. Say you ate one chocolate, thus you got 'x' unit of satisfaction. Then you ate another one and your level of satisfaction now reached to 'y' units. In this case, we define your marginal utility for chocolate to be $y-x$.
