Undefined functions + square roots of negative numbers

Simply put I understand the concept of functions and the concept of square roots of negative numbers/eulers notation. However, I have one problem I cannot make sense of that combines the two. I even have the solution but I do not understand the logic behind it. Any help would be wonderful.

Problem:

If f(x) = √((x^2)-4), then f(x) is undefined for which of the following values of x?

(A) -6

(B) -4

(C) -2

(D) 0

(E) 2

Solution Provided:

0≤x^2

4≤x^2

2≤x or -2≥x

"f(x) is undefined for any values of x that are not within the above range. Since 0 is not within the range, choice (D) is correct."

I must be missing something here. I know most of you are way over qualified to answer this question, but any clarification you can offer would be greatly appreciated.

Thanks