If d is the mean of x, y, and z, what is the mean of x + 5, y + 5, z + 5 ?

If we add or subtract a constant to each term in a set:
Mean will increase or decrease by the same constant.
SD will not change.
If we increase or decrease each term in a set by the same percent (multiply by a constant):
Mean will increase or decrease by the same percent.
SD will increase or decrease by the same percent.
So in our case mean will increase by 5 as we are adding 5 to each term in a set --> Mean =d + 5.
Answer: D.