It's not a perfect-square trinomial.
A perfect-square trinomial is of the form [tex]a^2 \pm 2ab+b^2=(a+b)^{2} [/tex].
If that looks confusing, just realize that the first and third terms need to be perfect squares (e.g., 16, because it is the square of 4, or 81, because it is the square of 9), and the middle term needs to be twice the product of a and b.
So [tex] x^{2} [/tex] is a perfect square equal to x times x. Is 20 a perfect square? Can you think of an integer whose square is 20? No, so 20 is not a perfect square. We are done.
The correct answer is D.
