Answer: Law of diminishing marginal utility
Explanation: In simple words, law of diminishing marginal utility states that as a consumer consume more of a good or service then the marginal benefit he or she receives from the additional consumption keeps on decreasing.
In the given case, Jenny's excitement keeps on decreasing with every chocolate she receives after a certain point of time.
Hence we can conclude that the given case illustrates law of diminishing marginal utility.
