A certain list consists of the first nine positive integers and an outlier. If the outlier is removed from the list, which of the following will always decrease?
The question is not concrete enough. Consider the list consisting of 1000 -10 s and 1000 10 s. Adding a 0 is an outlier but neither SD nor the range will decrease.
The question clearly says the list contains the first nine positive integers and an outlier. So I am not sure why you are assuming the list to be something else.