The correct response is when the data set is U-shaped.
In a U-shaped distribution, data points are concentrated at the ends of the range rather than in the center. This can lead to a situation where both the mean and median do not accurately represent the typical values of the dataset. The mean might be pulled towards the lower or upper end, while the median might fall in a middle range that doesn't reflect the clustering of values at the extremes. Therefore, neither the mean nor the median provides a good summary of the central tendency in a U-shaped distribution.