C. When there is no outlier, the mean is the appropriate measure of center.
When there are no outliers in a dataset, the mean is a good measure of center because it takes into account the values of all the data points. The median is also a good measure of center, but it may not be the best choice if there are extreme values or outliers in the dataset, as it can be influenced by those values. However, when there are no outliers, both the mean and the median are appropriate measures of center.
Option A is not true because the mean is not skewed in one direction when there is no outlier. Skewness refers to the shape of the distribution, and it can be affected by outliers, but not by the absence of outliers.
Option B is not true because the median is not skewed in one direction when there is no outlier. Skewness refers to the shape of the distribution, and the median is not affected by the shape of the distribution, but by the position of the values.
Option D is not true because the median can be used as the measure of center even when there is no outlier. It is a robust measure of center that is not influenced by extreme values.