Here, f(x) is a function if corresponding to every image there exist a unique pre-image.
So, f(x) is not a function if a an x value repeats.
Here, the other x values are 4, 9, 12.
If the 'a' is any of these x value f(x) is not a function.
Therefore, for f(x) to be not a function 'a' should be chosen as 4.
Hence, the correct option is first option.
