Correct Answer: **D.** *15.5*

If the mean of 15, x, 17 and 12 is 15, then

If the four values are arranged in ascending order (a descending order will also work), we have,

For an odd number of values, the one right in the middle would be the median. In this case, there is an even number of values; so we take the mean of the two values in the middle, i.e. of 15 an 16, which is 15.5.