Which statement below is incorrect?

the mean is not affected by the existence of an outlier.
the median is not affected by the existence of an outlier.
the standard deviation is affected by the existence of an outlier.
the interquartile range is unaffected by the existence of an outlier.

Well the median is not affected by the existence of an outlier because having 1,2,3   2 is the median but with and outlier such as 1,2,7 2 is still the median so that is correct 

the mean is not affected by the existence of an outlier well
the mean is an average of the number so for 1,2,3 the mean is 2 
now lets find the mean but with and outlier 1,2,7 the mean is 3.33 which is different from the first one we did so the mean IS affectes by the existence of an outlier

for short: the answer is THE MEAN IS NOT AFFECTED BY THE EXISTENCE OF AN OUTLIER