Answer: The correct answer is "e. occurs when there is a change in purchasing power as a result of a change in the price of a good.".
Explanation: The real-income effect occurs when there is a change in purchasing power as a result of a change in the price of a good.
The income effect corresponds to the variation in the quantity demanded of a good (or service) as a result of the modification of the purchasing power of purchase caused by a change in the price of the good in question.