Answer: D. 3
The case in which this function would be undefined would be when the denominator of the function is equal to zero. In order to make it zero in this situation, we need to set 27 - 9x equal to zero, 27 - 9x = 0.
We then want to isolate x terms, (27 - 9x) - 27 = 0 - 27, or -9x = -27.
Since both sides are negative, we want to multiply both sides by -1 so we can work with positive numbers, (-9x) * -1 = (-27) * -1, or 9x = 27.
We then divide by 9 to get our answer, 9x / 9 = 27 / 9, or x = 3.
Hope this helps, let me know if you have any further questions.