Concept: The question tests on the basic concepts of Statistics
The mean is the average of all the observations and depends on the sum of observations. When a value that increases the sum of observations is added, then the mean changes.
The standard deviation is the square root of the average of the sum of squares of the deviations from the mean. A value addition to one of the values, will change the deviations and hence the sum will also change. Therefore the standard deviation will also change.
The median is the middle value of the set of observations. Since 2.5 is added to the 10th observation, there is no shift in the median and will remain the same.