IMO its E.

because when u add the 6 and 6 to the list the average will not change but the standard deviation will come down.

take for example

5,10,6,7 the average is 7 and std deviation >0

If you were to add 2 numbers to this set, these numbers should
be closest to the mean so that their variance (number - mean) is 0
and the std dev reduces (because the total of variance for all the number in the set remains the same but its is divided by 5+2 in our example and 100+2 in question.)

hope it helps...